Treatment of BB rats with the plant alkaloid tetrandrine (20 mg·kg−1· day−1), a novel anti-inflammatory compound, from 35 to 120 days of age reduced the cumulative incidence of spontaneous diabetes from 75.5 to 10.9% (P < 0.001). Dose-response studies with 0, 5, 10, and 20 mg·kg−1·day−1 of tetrandrine from 35 to 130 days resulted in spontaneous diabetes in 84.2, 63.1, 31.6, and 5.3% of the rats, respectively. When the start of treatment with 20 mg·kg−1·day−1 tetrandrine was delayed until 70 days of age, there was a significant reduction of the incidence of diabetes from 63.1 to 28.6% (P < 0.01). Histological examination of the pancreases from tetrandrine-treated rats showed a lesser degree of insulitis than controls (P < 0.01). Drug toxicity was not seen in the rats, as assessed by appearance, behavioral change, organ histology, and blood chemistry. These results provide some hope that tetrandrine may be of value in preventing diabetes and treating newly diagnosed diabetic subjects, either by itself or in combination with a more potent immunosuppressive agent.