The epipodophyllotoxin derivative VM 26 inhibits entry of mouse mastocytoma cells into mitosis in cell cultures at drug concentrations of 0.01–1 μg/ml, the cells being arrested in G2 phase of the cell cycle. At higher concentrations, the compound exhibits spindle poison activity which manifests itself in a shortlasting rise in the number of cells arrested in metaphase of mitosis. These cells then disintegrate after a short period of time.