The effects of in vivo C3 depletion on the immune response were examined in rabbits by assaying for splenic PFC after immunizing normal or cobra venom factor-treated animals with aggregated human gamma-globulin. The response to this T-dependent antigen has previously been shown to be regulated such that several cycles of PFC appear following a single intravenous injection of antigen. C3 depletion had no effect on the first peak of PFC (appearing 5 days after injection), but resulted in depression of the second peak of PFC (day 13). In rabbits depleted of C3, antigen localization in splenic germinal centers was markedly decreased. Delaying C3 depletion until after antigen localization had occurred resulted in no depression of the second peak of PFC. These results suggest that one mechanism by which C3 affects immune responses in vivo is via its role in influencing the persistence of antigen. In the absence of C3, no significant localization of antigen occurs, resulting in interference with the cyclical production of antibody.