The new antibiotic FR-900482 (4-formyl-6,9-dihydroxy-14-oxa-1,11-diazatetracyclo-[7.4.1.02,7. O10,12]tetradeca-2,4,6-triene-8-ylmethyl carbamate) possesses an antitumor activity equal to or greater than that of mitomycin C (MMC). The hematotoxicity of equivalent effective doses of the two compounds was compared in mice. A single iv injection of either compound similarly decreased the number of white blood cells (WBC) in the peripheral blood, whereas FR-900482 had no effect on the number of platelets (PTL). Both drugs slightly reduced the number of red blood cells (RBC). The effect of FR-900482 on the bone marrow cells (BMC) measured by cfu in spleen and cfu in culture was weaker than that of MMC. The results suggest that FR-900482 is a promising antitumor agent both in efficacy and safety.