Effect of the immunosuppressive agent, ciclosporin (CS), on bovine serum albumin (BSA) nephritis in rats was evaluated. Eight weeks after immunization, 19 male Wistar rats received a daily intravenous dose of BSA (2 mg). Two weeks later, 11 rats received BSA and an oral dose of CS (10 mg/kg), and 8 rats received only BSA for 2 weeks. Urinary protein was measured weekly and serum anti-BSA antibody was measured by passive hemagglutination biweekly. The animals were killed at the 12th experimental week and blood samples and kidney specimens were obtained. BUN and serum creatinine were measured at the time of sacrifice. Kidney specimens were processed for light and immunofluorescent microscopic examination. Urinary protein excretion was significantly less in CS-treated rats than in nontreated controls at the 2nd week after treatment (5.3 ± 1.3 vs. 25.6 ± 10.3 mg/day, p < 0.05). Anti-BSA antibody titers were lower in treated rats than in controls at the 2nd week after the treatment. There were no significant differences in the levels of BUN and serum creatinine between two groups. Glomerular hypercellularity and mesangial widening were milder in treated rats than in controls, and glomerular deposition of BSA was less intense in treated rats than in controls. These results suggest that CS suppressed the antibody production and the development of glomerular changes in rats with immune complex glomerulonephritis.