diff --git a/specs/altair/beacon-chain.md b/specs/altair/beacon-chain.md
index 9b476bce1..f498c40c5 100644
--- a/specs/altair/beacon-chain.md
+++ b/specs/altair/beacon-chain.md
@@ -52,6 +52,7 @@
     - [Slashings](#slashings)
     - [Participation flags updates](#participation-flags-updates)
     - [Sync committee updates](#sync-committee-updates)
+- [Initialize state for pure Altair testnets and test vectors](#initialize-state-for-pure-altair-testnets-and-test-vectors)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 <!-- /TOC -->
@@ -661,3 +662,51 @@ def process_sync_committee_updates(state: BeaconState) -> None:
         state.current_sync_committee = state.next_sync_committee
         state.next_sync_committee = get_sync_committee(state, next_epoch + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
 ```
+
+## Initialize state for pure Altair testnets and test vectors
+
+This helper function is only for initializing the state for pure Altair testnets and tests.
+
+*Note*: The function `initialize_beacon_state_from_eth1` is modified: (1) using `ALTAIR_FORK_VERSION` as the current fork version, (2) utilizing the Altair `BeaconBlockBody` when constructing the initial `latest_block_header`, and (3) adding initial sync committees.
+
+```python
+def initialize_beacon_state_from_eth1(eth1_block_hash: Bytes32,
+                                      eth1_timestamp: uint64,
+                                      deposits: Sequence[Deposit]) -> BeaconState:
+    fork = Fork(
+        previous_version=GENESIS_FORK_VERSION,
+        current_version=ALTAIR_FORK_VERSION,  # [Modified in Altair]
+        epoch=GENESIS_EPOCH,
+    )
+    state = BeaconState(
+        genesis_time=eth1_timestamp + GENESIS_DELAY,
+        fork=fork,
+        eth1_data=Eth1Data(block_hash=eth1_block_hash, deposit_count=uint64(len(deposits))),
+        latest_block_header=BeaconBlockHeader(body_root=hash_tree_root(BeaconBlockBody())),
+        randao_mixes=[eth1_block_hash] * EPOCHS_PER_HISTORICAL_VECTOR,  # Seed RANDAO with Eth1 entropy
+    )
+
+    # Process deposits
+    leaves = list(map(lambda deposit: deposit.data, deposits))
+    for index, deposit in enumerate(deposits):
+        deposit_data_list = List[DepositData, 2**DEPOSIT_CONTRACT_TREE_DEPTH](*leaves[:index + 1])
+        state.eth1_data.deposit_root = hash_tree_root(deposit_data_list)
+        process_deposit(state, deposit)
+
+    # Process activations
+    for index, validator in enumerate(state.validators):
+        balance = state.balances[index]
+        validator.effective_balance = min(balance - balance % EFFECTIVE_BALANCE_INCREMENT, MAX_EFFECTIVE_BALANCE)
+        if validator.effective_balance == MAX_EFFECTIVE_BALANCE:
+            validator.activation_eligibility_epoch = GENESIS_EPOCH
+            validator.activation_epoch = GENESIS_EPOCH
+
+    # Set genesis validators root for domain separation and chain versioning
+    state.genesis_validators_root = hash_tree_root(state.validators)
+
+    # [New in Altair] Fill in sync committees
+    state.current_sync_committee = get_sync_committee(state, get_current_epoch(state))
+    state.next_sync_committee = get_sync_committee(state, get_current_epoch(state) + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
+
+    return state
+```
diff --git a/specs/altair/fork.md b/specs/altair/fork.md
index 2d9f2e26c..be562b82b 100644
--- a/specs/altair/fork.md
+++ b/specs/altair/fork.md
@@ -34,6 +34,8 @@ Warning: this configuration is not definitive.
 
 TBD. Social consensus, along with state conditions such as epoch boundary, finality, deposits, active validator count, etc. may be part of the decision process to trigger the fork. For now we assume the condition will be triggered at epoch `ALTAIR_FORK_EPOCH`.
 
+Note that for the pure Altair networks, we don't apply `upgrade_to_altair` since it starts with Altair version logic.
+
 ### Upgrading the state
 
 After `process_slots` of Phase 0 finishes, if `state.slot % SLOTS_PER_EPOCH == 0` and `compute_epoch_at_slot(state.slot) == ALTAIR_FORK_EPOCH`, an irregular state change is made to upgrade to Altair.
diff --git a/tests/core/pyspec/eth2spec/test/context.py b/tests/core/pyspec/eth2spec/test/context.py
index 002e84136..438e611cf 100644
--- a/tests/core/pyspec/eth2spec/test/context.py
+++ b/tests/core/pyspec/eth2spec/test/context.py
@@ -7,8 +7,8 @@ from eth2spec.utils import bls
 
 from .exceptions import SkippedTest
 from .helpers.constants import (
-    PHASE0, ALTAIR, MERGE, SHARDING, CUSTODY_GAME, DAS,
-    ALL_PHASES,
+    PHASE0, ALTAIR,
+    ALL_PHASES, FORKS_BEFORE_ALTAIR,
 )
 from .helpers.genesis import create_genesis_state
 from .utils import vector_test, with_meta_tags
@@ -55,17 +55,11 @@ class SpecForks(TypedDict, total=False):
 
 def _prepare_state(balances_fn: Callable[[Any], Sequence[int]], threshold_fn: Callable[[Any], int],
                    spec: Spec, phases: SpecForks):
-
-    p0 = phases[PHASE0]
-    balances = balances_fn(p0)
-    activation_threshold = threshold_fn(p0)
-
-    state = create_genesis_state(spec=p0, validator_balances=balances,
+    phase = phases[spec.fork]
+    balances = balances_fn(phase)
+    activation_threshold = threshold_fn(phase)
+    state = create_genesis_state(spec=phase, validator_balances=balances,
                                  activation_threshold=activation_threshold)
-    # TODO: upgrade to merge spec, and later sharding.
-    if spec.fork == ALTAIR:
-        state = phases[ALTAIR].upgrade_to_altair(state)
-
     return state
 
 
@@ -368,8 +362,6 @@ def with_configs(configs, reason=None):
 
 
 def is_post_altair(spec):
-    # TODO: everything runs in parallel to Altair.
-    #  After features are rebased on the Altair fork, this can be reduced to just PHASE0.
-    if spec.fork in [PHASE0, MERGE, SHARDING, CUSTODY_GAME, DAS]:
+    if spec.fork in FORKS_BEFORE_ALTAIR:
         return False
     return True
diff --git a/tests/core/pyspec/eth2spec/test/helpers/constants.py b/tests/core/pyspec/eth2spec/test/helpers/constants.py
index bffffe348..ccd7b20a2 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/constants.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/constants.py
@@ -18,7 +18,9 @@ DAS = SpecForkName('das')
 ALL_PHASES = (PHASE0, ALTAIR)
 # The forks that output to the test vectors.
 TESTGEN_FORKS = (PHASE0, ALTAIR)
-
+# TODO: everything runs in parallel to Altair.
+# After features are rebased on the Altair fork, this can be reduced to just PHASE0.
+FORKS_BEFORE_ALTAIR = (PHASE0, MERGE, SHARDING, CUSTODY_GAME, DAS)
 
 #
 # Config
diff --git a/tests/core/pyspec/eth2spec/test/helpers/fork_choice.py b/tests/core/pyspec/eth2spec/test/helpers/fork_choice.py
index 2ca37768b..f3b80b2ac 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/fork_choice.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/fork_choice.py
@@ -1,7 +1,5 @@
 from eth_utils import encode_hex
 
-from eth2spec.phase0 import spec as phase0_spec
-
 
 def get_anchor_root(spec, state):
     anchor_block_header = state.latest_block_header.copy()
@@ -58,8 +56,7 @@ def get_genesis_forkchoice_store(spec, genesis_state):
 
 def get_genesis_forkchoice_store_and_block(spec, genesis_state):
     assert genesis_state.slot == spec.GENESIS_SLOT
-    # The genesis block must be a Phase 0 `BeaconBlock`
-    genesis_block = phase0_spec.BeaconBlock(state_root=genesis_state.hash_tree_root())
+    genesis_block = spec.BeaconBlock(state_root=genesis_state.hash_tree_root())
     return spec.get_forkchoice_store(genesis_state, genesis_block), genesis_block
 
 
diff --git a/tests/core/pyspec/eth2spec/test/helpers/genesis.py b/tests/core/pyspec/eth2spec/test/helpers/genesis.py
index 14fd9aa47..49af43ec1 100644
--- a/tests/core/pyspec/eth2spec/test/helpers/genesis.py
+++ b/tests/core/pyspec/eth2spec/test/helpers/genesis.py
@@ -1,3 +1,7 @@
+from eth2spec.test.helpers.constants import (
+    ALTAIR,
+    FORKS_BEFORE_ALTAIR,
+)
 from eth2spec.test.helpers.keys import pubkeys
 
 
@@ -20,6 +24,11 @@ def create_genesis_state(spec, validator_balances, activation_threshold):
     deposit_root = b'\x42' * 32
 
     eth1_block_hash = b'\xda' * 32
+    current_version = spec.GENESIS_FORK_VERSION
+
+    if spec.fork == ALTAIR:
+        current_version = spec.ALTAIR_FORK_VERSION
+
     state = spec.BeaconState(
         genesis_time=0,
         eth1_deposit_index=len(validator_balances),
@@ -30,7 +39,7 @@ def create_genesis_state(spec, validator_balances, activation_threshold):
         ),
         fork=spec.Fork(
             previous_version=spec.GENESIS_FORK_VERSION,
-            current_version=spec.GENESIS_FORK_VERSION,
+            current_version=current_version,
             epoch=spec.GENESIS_EPOCH,
         ),
         latest_block_header=spec.BeaconBlockHeader(body_root=spec.hash_tree_root(spec.BeaconBlockBody())),
@@ -47,8 +56,19 @@ def create_genesis_state(spec, validator_balances, activation_threshold):
         if validator.effective_balance >= activation_threshold:
             validator.activation_eligibility_epoch = spec.GENESIS_EPOCH
             validator.activation_epoch = spec.GENESIS_EPOCH
+        if spec.fork not in FORKS_BEFORE_ALTAIR:
+            state.previous_epoch_participation.append(spec.ParticipationFlags(0b0000_0000))
+            state.current_epoch_participation.append(spec.ParticipationFlags(0b0000_0000))
+            state.inactivity_scores.append(spec.uint64(0))
 
     # Set genesis validators root for domain separation and chain versioning
     state.genesis_validators_root = spec.hash_tree_root(state.validators)
 
+    if spec.fork not in FORKS_BEFORE_ALTAIR:
+        # Fill in sync committees
+        state.current_sync_committee = spec.get_sync_committee(state, spec.get_current_epoch(state))
+        state.next_sync_committee = (
+            spec.get_sync_committee(state, spec.get_current_epoch(state) + spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
+        )
+
     return state
diff --git a/tests/formats/README.md b/tests/formats/README.md
index 8f817220e..16f756fc1 100644
--- a/tests/formats/README.md
+++ b/tests/formats/README.md
@@ -139,6 +139,8 @@ E.g. `pre.ssz_snappy`, `deposit.ssz_snappy`, `post.ssz_snappy`.
 Diffing a `pre.ssz_snappy` and `post.ssz_snappy` provides all the information for testing, when decompressed and decoded.
 Then the difference between pre and post can be compared to anything that changes the pre state, e.g. `deposit.ssz_snappy`
 
+Note that by default, the SSZ data is in the given test case's <fork or phase name> version, e.g., if it's `altair` test case, use `altair.BeaconState` container to deserialize the given state.
+
 YAML is generally used for test metadata, and for tests that do not use SSZ: e.g. shuffling and BLS tests.
 In this case, there is no point in adding special SSZ types. And the size and efficiency of YAML is acceptable.