Summary Hemagglutination is of questionable validity for following the synthesis of 19S to 7S chicken anti-BSA antibodies, since only small amounts of the major antibody population (7S) are measured in early primary antisera. The 7S antibodies in primary antisera were demonstrated by antigen-binding methods (ammonium sulfate salting out procedure and RIE). Radioimmunoelec-trophoretic patterns localized anti-BSA and anti-HSA primary antibody activities in the 7S γ−, γ1A−, and γ1M−globulins; however, only a few of these sera had γ1A− activity. No γ1M− antibody was detected by RIE in hyperim-mune antisera, All anti-azobenzoate antisera had γ− and γ1M−globulin antibodies, but no activity in the γ1M−globulin. The species of chicken antibody synthesized seems to be related to the nature of the immunizing antigen.