Japan White rabbits were treated with aclacinomycin A, a new anthracycline antitumor antibiotic, at a dose of 6.25 or 25.0 mg/kg by single intravenous, or 12.5 or 50.0 mg/kg by single oral administration, respectively. Beagle dogs were treated at a dose of 3.0 or 6.0 mg/kg by single intravenous injection. In rabbits in higher dose groups, RBC and WBC counts as well as lymphocyte ratio in peripheral blood decreased on day 1. Nucleated cell counts and erythroid elements in bone marrow decreased to raise M/E ratio (Myeloid/Erythroid ratio) on day 3. In a dog given at 6.0 mg/kg, WBC and platelet counts, lymphocyte and neutrocyte per cents in peripheral blood and also nucleated cells, particularly erythroid elements in bone marrow remarkably decreased on day 3 accompanied with an increase in M/E ratio. These changes were almost completely recovered by day 14 in both animals. No abnormalities were found in lower dose groups. Male Wistar rats, treated with the drug at a dose of 1.5 mg/kg by daily intraperitoneal injection for 30 days, showed slight decreases in peripheral WBC and RBC counts and M/E ratio in bone marrow. No change was observed in rats treated at 0.75 mg/kg and less for 30 days.