RP 56976 (taxotere), a new semisynthetic analogue of taxol, is a potentially important chemotherapeutic agent for the treatment of cancer. We report here that this drug is a potent inhibitor of cell replication and, like taxol, promotes the in vitro assembly of stable microtubules in the absence of guanosine triphosphate and induces microtubule-bundle formation in cells. Compared with taxol, RP 56976 is slightly more active as a promoter of tubulin polymerization. As an inhibitor of cell replication, RP 56976 is 2.5-fold more potent than taxol in J774.2 and P388 cells and at least 5-fold more potent in taxol-resistant cells.