Effects of acetoside (ACT) on crescentic-type anti-GBM nephritis in rats were investigated. When rats were treated with ACT from the 1st day after i.v. injection of anti-GBM serum, ACT inhibited the elevation of protein excretion into urine. In the ACT-treated rats, cholesterol and creatinine contents and antibody production against rabbit gamma-globulin in the plasmas were lower than those of the nephritic control rats. Histological observation demonstrated that this agent suppressed hypercellularity and the incidence of crescent formation, adhesion of capillary wall to Bowman's capsule and fibrinoid necrosis in the glomeruli. Furthermore, rat-IgG and C3 deposits on the GBM were significantly less in the ACT-treated group than in the control nephritic group. When the treatment was started from the 20th day after i.v. injection of anti-GBM serum, by which the disease had been established, ACT resulted in a similar effect on the nephritic rats as stated above. These results suggest that ACT may be a useful medicine against rapidly progressive glomerulonephritis, which is characterized by severe glomerular lesions with diffuse crescents.