Wistar rats, both male and female, were treated with aclacinomycin A at 4 dosage levels (0.375, 0.75, 1.5 and 3.0 mg/kg/day) by daily intraperitoneal injection for 30 days. Several rats died in the two higher dose groups (1.5 mg/kg/day: male 2/8 and 3.0 mg/kg: male 8/8, female 8/8. Piloerection, anorexia, depression of spontaneous activity, diarrhea and slight incontinence were observed in rats in 3.0 mg/kg/day dose group. Body weight gain decreased after day 3 in rats receiving 1.5 and 3.0 mg/kg/day. A significant decrease in the total WBC count and a slight decrease in RBC count were observed in animals of 1.5 mg/kg/day dose group which were survived for 30 days. Autopsy findings demonstrated atrophy of the thymus and spleen, and hyperemia and hemorrhage in the intestine. The atrophy of the thymus and decreased hematopoiesis in the bone marrow were histologically noted in the two highest dose groups. No cardiotoxicity was observed.