The effects of fumonisin B1 (FB1) intoxication in chickens was evaluated in three experiments. Two-day-old broiler chicks were fed a diet containing 10 mg pure FB1/kg feed for 6 days; some chicks were necropsied at this time, and others were allowed to recover for 5 weeks before necropsy. In two other experiments, 2-day-old chicks were fed a broiler starter ration prepared with Fusarium moniliforme culture material containing FB1; one group received 30 mg/kg for 2 weeks, and another received 300 mg FB1/kg for 8 days. Compared with controls, intoxicated chicks exhibited diarrhea; decreases in body weight and in liver, spleen, and bursa absolute weights; a hepatic relative weight increase; and spleen relative weight decrease. Triglycerides, uric acid levels, and alkaline phosphatase activity decreased, and gamma glutamyl transferase, aspartate aminotransferase, lactic dehydrogenase, creatine kinase, and cholesterol increased. The results indicate that low doses of pure FB1 (10 mg/kg) and FB1 from Fusarium moniliforme culture material (30 mg/kg) are toxic to young chicks.