Isohemagglutinins were detected in pure γA-globulin (IgA), γM-globulin (IgM) and γG-globulin (IgG) fractions of sera from normal and immunized human subjects. The anti-A isoagglutinin in the immune sera was associated with both polymer and monomer forms of IgA. On a weight basis, the polymer form IgA antibody and IgM antibody had comparable hemagglutinating activities. The hemagglutinating activity of the monomer form IgA antibody was lower than that of the polymer form antibody, but higher than that of IgG antibody. The hemagglutinating activity of both polymer and monomer forms of IgA isoagglutinins was markedly diminished by reduction and alkylation. In contrast to the IgG and IgM isoagglutinins, the IgA isoagglutinins did not hemolyze homologous red cells with either guinea pig or human C′. The IgA isoagglutinin did not sensitize guinea pig skin for passive cutaneous anaphylaxis.