The BB rat spontaneously develops insulin-dependent diabetes mellitus (IDDM) in association with marked insulitis in the islet of Langerhans. Since platelet-activating factor (PAF-acether) is involved in allergic and inflammatory reactions, we tested a PAF antagonist, Ginkgolide B (BN 52021) for potential effects on islet inflammation and diabetes. Diabetes prone BB/Wor rats were treated daily from weaning at 25 days until 105 days of age with either saline (n = 30, controls), 10 (n = 25, low dose) or 20 (n = 30, high dose) mg/kg body weight of BN 52021. The overall incidence of IDDM was unaffected by treatment. Quantitative analysis of insulin area showed a dose-dependent protection of beta cells by Ginkgolide B, reflected in a 6- (low dose) to 8-fold (high dose) (P less than 0.01-0.005) increase in the insulin/glucagon cell ratio compared to the saline treated rats. Ginkgolide B reduced severe insulitis from 84% in the saline rats developing IDDM to 59% (n.s.) in the low and to 33% (P less than 0.001) in the high dose group. These data suggest that PAF inhibitors may prove useful in immunomodulator therapy of IDDM since beta cells are preserved.