Guinea pig peritoneal macrophages produced superoxide anions (O2-) when reacted with ovalbumin complexes of homologous IgG1 and IgG2 antibodies. In this reaction, IgG2 complexes were about three times as active as IgG1 complexes. But the susceptibility of IgG1 complexes to phagocytosis by the cells appeared to be indistinguishable from that of IgG2 complexes. The avidity of IgG1 complexes in the antigen excess zone for Fc receptors on the cells was lower than that of the IgG2 counterparts. The amount of IgG1 complex bound to the cells, however, did not significantly differ from that of IgG2 complexes when compared using each complex at the equivalence zone which showed maximal effector functions on the cells. The binding of Clq to IgG2 complexes increased markedly the amounts of complexes bound to the cells, but it reduced O2- generation. These results suggest that the difference in abilities of IgG1 and IgG2 complexes to promote O2- generation may be caused by different structures of the Fc parts or their antigen complexes involved in priming macrophages for O2- generation.