Kazusamycin B is a minor component of the leptomycin complex produced from the fermentation of the broth of a|Streptomyces sp.|. The compound is a hydroxylated analogue of Leptomycin A , a nuclear export inhibitor, and highly lipophillic.|In vitro|antitumor activity of Kazusamycin B on tumors L1210, P388/S, B16, HeLa S3, L5178Y-ML or L929 was ascertained to have an IC|50|of 1.8, 2.0, 0.7, 1.8, 10.0 and l.0 ng/ml when cultured in the presence of Kazusamycin B for 72 hours. The compound has also displayed broad antitumor activity in mice with effectiveness being more dependent on dosing regimen than dosing concentration both|in vivo|and|in vitro|. Kazusamycin B also possesses antibacterial and antifungal activity.