Mutants of Chinese hamster ovary (CHO) cells independently selected for resistance to the anticancer drug 4'-demethylepipodophyllotoxin thenylidene-beta-D-glucoside (VM-26) and the microtubule inhibitor podophyllotoxin exhibit mutually an exclusive cross-resistance pattern toward various podophyllotoxin derivatives that possess either VM-26-like or podophyllotoxin-like activities, respectively. The cross-resistance studies with these mutants have led to the identification of a new podophyllotoxin derivative, 4'-demethylepipodophyllotoxin-beta-D-glucoside (compound No. 13), which possesses VM-26-like and 4'-demethylepipodophyllotoxin ethylidene-beta-D-glucoside (VP-16-213)-like activity. This inference is strongly supported by our observation that, like VM-26 and VP-16-213, compound No. 13 showed no antimitotic activity (as seen by the effect on the mitotic index) but was highly active in inducing DNA strand breaks, sister chromatid exchanges, and mutations at the hypoxanthine-guanine phosphoribosyl transferase and adenosine kinase loci in CHO cells. On the basis of structure-activity data of various podophyllotoxin derivatives, three structural features have been identified that appear essential for VM-26-like activity. These are: 1) the presence of an hydroxyl group at the 4'-carbon, 2) an epi-configuration at the C-4 position, and 3) the presence of a glycoside moiety on the hydroxyl group at the C-4 position. In addition, the nature of the glycoside substituents at the C-4 position greatly affects the relative VM-26-like activity of various compounds.