The present study was designed to evaluate the therapeutic activity of a novel immunosuppressive agent, deoxyspergualin (DSG, NKT-01) in male MRL/MpJ-lpr/lpr (MRL/lpr) mice suffering advanced systemic lupus erythematosus (SLE)-like lesions. Treatment with DSG in the early phase of the disease at doses of 1.5 and 3mg/kg strongly suppressed the development of SLE-like lesions. When DSG was administered from week 21 through 29 to MRL/lpr mice in advanced phases of the disease, a daily iv dose of 3 mg/kg (5 days/week) markedly reduced the symptoms, whereas a dose of 1.5 mg/kg did not. Moreover, DSG treatemt at a dose of 3mg/kg, started at the time when the blood urea nitrogen levels were over 50 mg/deciliter, significantly prevented deterioration of the hyperuremia. Taking these findings into consideration, DSG was found to be a promising agent for curing such established autoimmune disease.