The immunological reactions of calves to inoculation of plasma obtained from other calves infected with Babesia argentina were studied to ascertain the general nature of the antigen contained and its possible role in the development of acquired immunity. Three types of antigen were recognized. The first was an autoantigen, associated with polymeric haptoglobin complexes. It caused the formation of precipitating antibodies that were not specific for babesial infection and appeared to have no functional significance in the development of acquired immunity to the disease. The second group caused the production of lysins for homologous but not autologous erythrocytes. They were cattle blood-group antigens, released by erythrocyte destruction during infection. Their functional significance in acquired immunity was also doubtful. The third type probably originated in the parasite, caused the formation of antibodies specific for B. argentina, and induced partial protection in noninfected splenectomized calves. This protection was directed against different strains of B. argentina and was chiefly manifest by a delay in the time required for the parasites to reach detectable levels in peripheral blood. Complete protection, characterized by abrupt removal of parasites from the peripheral blood, was observed in only one experiment in which the parasites used to challenge immunity were antigenically identical with those that produced the infected plasma for immunization. These findings were discussed in relation to artificial immunization.