A single administration of 1 X 10(9) heated erythrocytes to C57BL and BALB/c mice caused on the 13th day the appearance of antierythrocytic autoantibodies, an increase in the weight of the lymphoid organs, and lymphoreticular hyperplasia. These changes were more pronounced in BALB/c mice. During the development of autoimmune reactions the changes in the number of E- and EAC-rosette-forming cells in the thymus and spleen and in the immune response to the sheep erythrocyte immunization and E. coli endotoxin were revealed; distinct strain differences were observed. Daily somatotropic hormone administration (5 mg/kg of body weight) for 10 days decreased the degree of the autoimmune reactions development in mice of both strains. Its action was more expressed in BALB/c mice.