General Antonyms:
* Cooperative
* Collaborative
* Harmonious
* Friendly
* Supportive
* Synergistic
* Altruistic
* Beneficial
* Concordant
More Specific Antonyms, Depending on Context:
* Complementary (if referring to opposing forces that work together)
* Supportive (if referring to a relationship where one person supports the other)
* Concordant (if referring to agreement or harmony)
Example:
Instead of "antagonistic" behavior, you could say "cooperative" behavior or "harmonious" relationship.
The best antonym will depend on the specific context in which you are using the word "antagonistic."