A new antitumor substance, BE-13793C, which has topoisomerase inhibitory activity was isolated from the culture broth of a strain of actinomycetes. The producing strain, BA13793, isolated from a soil sample collected in Seto, Aichi Prefecture, Japan, has a resemblance to Streptoverticillium mobaraense. The active principle was extracted from the mycelium of strain BA13793 with methanol and purified by Sephadex LH-20 column chromatography. BE-13793C showed strong inhibitory activity against topoisomerases I and II and inhibited the growth of doxorubicin-resistant or vincristine-resistant P388 murine leukemia cell lines, as well as their parent P388 cell line.