The effect of platelets on the development of immune complex glomerulonephritis (GN) was examined using bovine serum albumin (BSA) GN with platelet depletion. To clarify the role of platelets in the initial stage of BSA GN, thrombocytopenia was induced before BSA infusion. In 18 New Zealand white rabbits, BSA was intravenously injected twice after the presensitization. Eight of these BSA GN rabbits were injected daily with goat anti-rabbit platelet antiserum to induce thrombocytopenia, and platelet counts were maintained below 5 × 104/μl throughout the experiment. In the thrombocytopenic group, the degree of proteinuria was significantly decreased compared to the control group. Glomerular polymorphonuclear leukocyte infiltration, mononuclear cell proliferation, exudation and glomerular enlargement were significantly suppressed in the thrombocytopenic group. The results suggest that platelets may be quite important in the initiation and development of immune complex