The immune competence of two species of marmosets, S. fusciollis and S. oedipus, was evaluated by the intravenous (i.v.) and intramuscular (i.m.) injection of sheep red blood cells (SRBC). In S. fusciollis marmosets, 1 ml of a 50% suspension yielded titres of haemolysin and agglutinating antibodies equal to or greater than 1 ml of a 10% dose of antigen. In both species, the i.v. route, while resulting in formation of 19S and 7S agglutinins, yielded only 19S haemolysins, even after multiple antigen injections. Repeated i.v. injections resulted in a progressive decrease in peak titres, in contrast to the i.m. route, where booster inoculations gave a typical anamnestic response. Jerne plaque-forming cells (PFC) in the spleens of S. oedipus marmosets showed predominately 19S plaques after a primary i.v. challenge; only 19S PFC were detected in the spleen of an animal that had been given multiple inoculations, the type of antibody produced reflecting that found in the serum. 19S but not 7S haemolysins of both species were sensitive to heating at 56 degrees C for 1/2 hr. The serum titres and splenic PFC data from the marmosets suggest these animals, particularly S. oedipus, respond poorly to SRBC when a comparison is made to similar studies in mice and rats.