Autoimmune female New Zealand Black/New Zealand White mice were treated with frentizole, an experimental immunosuppressive drug. Three groups of "young," 8-week-old mice received high-dose frentizole (80-84 mg/kg/day), low-dose frentizole (8 mg/kg/day), or no drug (controls); these mice were followed until spontaneous death. Three groups of "old," 24-week-old mice with established lupus-like disease were treated with high or low doses of frentizole. Old control mice received no drug. After 12 weeks of therapy, surviving old mice were killed. Beneficial therapeutic response was achieved when high-dose treatment was started at an early age; antiDNA values were suppressed, and longevity was prolonged significantly. Frentizole did not arrest the progression of renal disease in old mice. Glomerulonephritis and vasculitis were the most common causes of death in young and old animals. Twenty-nine percent of young, high-dose-treated mice died with neoplasms. Large glomerular deposits of IgG, IgM, and C3 were present in renal tissue from treated and control mice. Peripheral lymphocyte counts and mitogenic responses of spleen cells were not changed by treatment. The efficacy of frentizole in a murine model of lupus supports its usefulness as an immunoregulatory drug.