Two major histocompatible strains of rats differing at 'minor' antigens including immunoglobulin allotype disparity were used in experiments to study antibody-mediated suppression of a primary antibody response against third party allogeneic red cells. Antibodies against red cells were raised by repeated injections in the two strains of rats, and the effects of the two pools of serum to suppress antibody response upon intravenous injection of a small test dose of blood were assessed and compared in in vitro titrations. It was concluded that allotype differences, as suggested by others, were not necessary and they were not amplifying the suppression. The observations are discussed in relation to the Rh immunoprophylaxis in man.