Immune serum delayed the onset of parasitemia in both intact and splenectomized mice, but it neither prevented the development of Babesia rodhaini infection nor protected the mice from death even with further supplementation of immune serum during the infection. The protective antibodies in the serum are more effective in their action on free B. rodhaini parasites than on infected erythrocytes; the parasites (free or inside the red cells) being direct targets for the antibodies. Passive administration of the immune serum seemed to inhibit the active development of the host's own immune response. B. rodhaini, unlike Plasmodium berghei, produces surface changes in the membranes of infected erythrocytes and therefore on incubation of infected erythrocytes and therefore on incubation of infected erythrocytes in immune serum there is either entry and subsequent interaction of antibodies with the parasites within the erythrocytes or there is action of antibody with antigen in the erythrocyte membranes. The effect of immune serum on parasitized erythrocytes is greatest when the parasitized cells incubated in immune serum were administered with additional amounts of immune serum, indicating the ability of the immune serum to extend its action in vivo.