Kazusamycins A and B and leptomycin B have a structure characteristic of an unsaturated, branched-chain fatty acid with a terminal delta-lactone ring, and show antibacterial activity on some kinds of fungi. Kazusamycin A (KZM-A) showed cytotoxic activity on mammalian cells at very low concentrations (ng/ml) in vitro. The antibiotic inhibited not only the growth of transplantable murine tumors and their metastases to the lung but also human mammary tumors inoculated into nude mice. KZM-A became immediately distributed to the main organs of mice, and a certain quantity of the antibiotic was inactivated by binding to high-molecular-weight substances such as albumin. A large quantity of KZM-A was carried to the liver and excreted into the bile, but was then reabsorbed by the small intestine. The growth of tumor metastases (L5178Y cells) in the liver was suppressed by KZM-A. The antibiotic induced severe diarrhea by causing necrosis and/or lysis of the mucous membrane of the small intestine. In contrast to this, the degree of myelotoxicity was relatively slight. The active site of the fatty acid of KZM-A appeared to consist of conjugated double bonds, carboxylic acid and hydroxyl moieties.