Complement fixing capacity, antibody-dependent cell cytotoxicity and in vivo opsonic function of rabbit anti-DNP precipitating and co-precipitating antibodies were analysed. Precipitating antibody activated the complement system, but co-precipitating antibody did not. Both antibody preparations behaved similarly in antibody-dependent cytotoxicity reactions. When chicken erythrocytes were employed as target cells, both antibodies were inactive with homologous (rabbit) lymphocytes as effector cells but were active with heterologous lymphoid effector cells (human, guinea-pig). Clearance of antigen from the blood of mice was accelerated following injection of an optimal dose of precipitating antibody but co-precipitating antibody was ineffective even at different doses. Competition between precipitating and co-precipitating antibodies in complement fixation and antigen elimination following interaction with antigen successively or at the same time was investigated. In these tests, both antibodies competed with antigen by mass but not by affinity. The possibility exists that co-precipitating antibody has one high and one low affinity binding site and thus acts as an univalent blocking antibody and is therefore not able to form active complexes with antigen. The possible role of co-precipitating antibody in some humoral and cellular immune mechanisms is discussed.