Histologic examination of kidneys from rats infected with the hemosporidian protozoan Babesia rodhaini demonstrates the presence of a moderate, acute proliferative glomerulitis. Immunofluorescent studies establish that this renal complication is associated with glomerular deposits of IgG and the third component of complement (C3) in a pattern that is diagnostic for soluble, immune complex-induced nephritis. Acid elution of proteins from involved kidneys leads to the recovery of IgG that is reactive with babesial antigen. In the course of rat babesiosis hypocomplementemia develops, coincident with the parasitemia, severe anemia and glomerular deposits of IgG and C3.