diff --git a/langchain/chat_models/openai.py b/langchain/chat_models/openai.py
index f6e0f381..45a495af 100644
--- a/langchain/chat_models/openai.py
+++ b/langchain/chat_models/openai.py
@@ -128,7 +128,7 @@ class ChatOpenAI(BaseChatModel, BaseModel):
     """Whether to stream the results or not."""
     n: int = 1
     """Number of chat completions to generate for each prompt."""
-    max_tokens: int = 256
+    max_tokens: Optional[int] = None
     """Maximum number of tokens to generate."""
 
     class Config: