Kazusamycin B is a bacterial metabolite originally isolated from Streptomyces. It has antifungal activity against S. pombe and R. javanicus (MICs = 0.05 and 2.13 µg/ml, respectively) but is inactive against Gram-positive and Gram-negative bacteria (MICs = >100 µg/ml). Kazusamycin B (5 ng/ml) halts the cell cycle at the G1 phase and induces nuclear condensation in L1210 cells, as well as inhibits nuclear-to-cytosolic transport of the HIV-1 regulatory protein Rev in HeLa cells expressing Rev (IC50 = 6.3 nM). It is cytotoxic to L1210 and HCT-8 cells with IC50 values of 1.8 and 1.6 ng/ml, respectively, and reduces tumor growth in a variety of murine tumor and mouse xenograft models.