Summary: The inhibitor of influenza virus strain A/Asia/57 possesses antigenic activity. By immunizing chickens, antisera can be obtained which specifically neutralize the inhibitor. The titer of the inhibitor neutralizing antibodies in the anti-inhibitory chicken serum shows no correlation with the titer of the antihorse antibodies in that serum. Within the range of dilutions in which the anti-inhibitory activity of hen serum is manifest, the intrinsic inhibitor of the chicken serum does not exhibit itself.