High levels of immunity to Anaplasma marginale were induced in cattle either by vaccination using sonically disrupted A. marginale-infected erythrocytes or by repeated infection with different strains of the rickettsia. In both instances, high levels of anti-A. marginale antibody were detected in the sera of the immune cattle by immunoblotting. Serum from one animal that had been made immune by repeated infection was transferred intravenously to A. marginale-susceptible calves (three non-splenectomised and two splenectomised) undergoing initial A. marginale infection at serum doses of 2-10 ml/kg. Neither the course nor the outcome of infection as indicated by the parasite levels attained or the level of anaemia induced was altered in the calves that received the immune serum relative to the course or outcome of infection in control calves (two non-splenectomised and two splenectomised) that received serum from an two splenectomised) that received serum from an A. marginale-naive donor animal. In a similar experiment, a pool of sera from four steers that had been vaccinated with sonically disrupted A. marginale initial bodies was transfused into two intact A. marginale-susceptible calves during the early stage of A. marginale infection at a dose of 10 ml/kg. No difference was observed in the course or outcome of infection in these calves relative to the course or outcome of infection in the two non-splenectomised calves that were transfused with non-immune serum.(ABSTRACT TRUNCATED AT 250 WORDS)