Complement has been shown to affect the solubility of antigen-antibody complexes by two mechanisms: in the first, classical pathway dependent, complement inhibits the formation of the immune precipitate; in the second, alternative pathway dependent, complement reacts with a formed precipitate to bring about its solubilization. The biological properties of complement reacted immune complexes (IC) has been assessed by studying their binding to staphylococcus protein A (SPA) and to human erythrocytes. BSA-anti-BSA complement reacted IC bound to human erythrocytes and to SPA. Complexes generated by solubilization of immune precipitates showed greater immune adherence than complexes held in solution by complement, despite their similar size. Complexes held in solution in a factor D depleted human serum bound more efficiently to erythrocytes than complexes formed in normal serum. These experiments demonstrate that complement reacted IC cannot be regarded as biologically inert and that factors affecting complement function may have important effects on the properties of antigen-antibody complexes.