Experiments were undertaken to study whether progressive processes of bovine serum albumin (BSA) nephritis in C57BL/B10-BR mice could be prevented by the treatment with methylprednisolone (MPSL), a glucocorticoid. Crescentic glomerulonephritis could be in duced in 22 mice with high 95 per cent reproducibility by preimmunization of 0.2 mg of BSA four times every 2 weeks, follwed by daily intraperitoneal injections of 50 mg/kg of BSA for 4 weeks (group IV, a control). In groups of 8 mice each, intraperitoneal injection of 50 mg/kg of MPSL was initiated 3 days before premmunization (group I), before daily intraperitoneal injection of BSA (group II), and after daily intraperitoneal injection of BSA (group II), and after daily intraperitoneal injection of BSA (group III). In each group, the treatment was continued until the final day of the experiment. In group I, anti-BSA antibody titers, the levels of circulating immune complexes (CIC) and urinary protein were significantly suppressed in comparison with those in group IV. Histologically, diffuse intra-and extra-capillary proliferation observed in group IV was appar ently inhibited. In group II and III, urinary protein and intra-/extracapillary proliferation were significantly diminished compared with those of group IV in spite of minor or no decrease in antibody titers and the CIC levels. These results suggest that MPSL has a suppressive effect on the progressive processes of immune complex mediated glomerulonephritis through both its immunosuppressive and antiinflammatory actions, especially when used in the early stage of immune reaction.