New antitumor substance, designated BE-18591, was isolated from the culture broth of a streptomycete strain BA18591, which was isolated from a plant sample collected in Hamamatsu, Shizuoka Prefecture, Japan during a screening program for new antitumor substances and classified as Streptomyces sp. The active principle was extracted from the mycelium with methanol and purified by silica gel column chromatography. BE-18591 showed cytotoxic activity against MKN-45 human stomach cancer cell line and P388 murine tumor cell line. In in vivo experiments, it inhibited the growth of Ehrlich ascites tumor. Additionally, BE-18591 exhibited antimicrobial activity against Gram-positive and some Gram-negative bacteria. This paper describes the isolation, physico-chemical properties and biological activities of BE-18591, while its structure elucidation is reported in an accompanying paper. The producing strain has been deposited at the National Institute of Bioscience and Human-Technology (Japan) with access number FERM-P11437, and taxonomic studies of the strain will be reported separately.