The immune response to sheep erythrocytes was compared in mice treated with cobra venom (CoF) and with pneumococcal capsular antigen S III. Suppression of the immune response (in both cases) was attributed to the ability of both tested substances to activate the 3rd component of complement. CoF and S III both changed the organ localization of labeled antigens capable of activating complement. No evidence for an alternative pathway by which S III could activate complement was found. CoF and S III were compared in the light of their ability to suppress the immune response.