The Vinca alkaloids differ in their [cancer] chemotherapeutic effectiveness and toxicities. To determine whether differences are due to a differential effect on the assembly of tubulin into microtubules, the effects of vincristine, vinblastine and a newer alkaloid, desacetyl vinblastine amide, were examined on the assembly of bovine brain tubulin in vitro. The 3 compounds block bovine tubulin polymerization in vitro and almost equally effectively at a 1 .mu.M concentration (tubulin, 6.5 .mu.M). At 10 .mu.M, the 3 alkaloids also interact with preformed microtubules in vitro, causing spiral-like distortions of the microtubules. No effect of the alkaloids on polymerization of another fibrous protein, actin, was observed. The differential actions of vinblastine, vincristine and desacetyl vinblastine amide in vivo seem to be based on some biological process other than the reaction with tubulin or the microtubules per se.