Numbers of direct and indirect plaque-forming cells (pfc) per 106 spleen cells, total and mercaptoethanol-resistant hemolysin and hemagglutinin activity have been determined in adult male Syrian hamsters at different times after primary and secondary intraperitoneal injection of 1×1010 sheep red blood cells (SRBC). Only a high antigen dose was sufficient for stimulating a uniform immune response. The kinetics of the primary response at the cellular and serum level corresponds in general to that of the mouse and other laboratory animals. In the secondary response the direct pfc/106 spleen cells and total serum hemolysin and hemagglutinin concentrations were higher than in the primary response and the pfc and serum antibody curves declined rapidly again after reaching their peaks.