The therapeutic efficacy of immunosuppressants for treating rapidly progressive glomerulonephritis (RPGN) with crescent formation remains controversial. SCG/Kj mice spontaneously develop RPGN-like symptoms, characteristic of crescentic glomerulonephritis and systemic small vessel vasculitis, associated with the presence of anti-neutrophil cytoplasmic antibodies (ANCA). We evaluated the ''ameliorative'', not prophylactic, effects of immunosuppressive agents, deoxyspergualin (DSG), cyclophosphamide (CYC) and prednisolone (PDN), on RPGN in these mice. DSG at intraperitoneal doses of 3 and 6 mg/kg, CYC at an oral dose of 12 mg/kg, or PDN at an intraperitoneal dose of 120 mg/kg was administered once a day for 21 days to female mice ''at the onset of hematuria''. A set of control SCG/Kj mice received only saline injections. DSG and CYC significantly prolonged survival, improved the proteinuria, hematuria and hyperuremia, and decreased the serum level of myeloperoxidase-ANCA. Moreover, DSG significantly suppressed the formation of crescents in glomeruli. PDN failed to affect any of the parameters. DSG might be useful for inducing remission in crescentic glomerulonephritis involved in RPGN.