Experiments were undertaken to clarify whether a large dose of methylprednisolone (MPSL) could have any suppressive effect on progressive Masugi nephritis in the rabbit. Progressive crescentic Masugi nephritis could be induced with high reproducibility by preimmunization with a small amount of nephrotoxic duck gamma-globulin incorporated with complete Freund's adjuvant, followed by an intravenous injection 4 days later. Two groups of rabbits treated with 80 mg/kg of MPSL either before or after the development of proteinuria, showed a significant decrease in both antibody titers and serum creatinine levels during treatment. Histologically, the prominent diffuse intracapillary proliferation and crescent formation observed in controls, were markedly diminished. Accumulations of monocytes in the intra- and extracapillary space were also decreased. These results suggest that suppression of antibody production by a large dose of MPSL is one of its most fundamental actions, and can prevent the processes leading to crescentic glomerular lesions.