Vinepidine, a new derivative of vincristine, and three clinically used Catharanthus derivatives, vinblastine, vincristine, and vindesine, were examined for their abilities to inhibit net tubulin addition at the assembly ends of bovine brain microtubules at steady state. Although all four derivatives were generally similar in potency, their relative abilities to inhibit tubulin addition were distinguishable. Vinepidine and vincristine were the most potent derivatives (K_i, 0.079 ± 0.018 (SD) µm and 0.085 ± 0.013 µm, respectively), followed by vindesine (K_i, 0.110 ± 0.007 µm) and vinblastine (K_i, 0.178 ± 0.025 µm). In contrast to their relative abilities to inhibit microtubule assembly in vitro, vinblastine and its derivative, vindesine, were generally more potent than vincristine and vinepidine in inhibiting cell proliferation in culture. Vinblastine was nine times more potent than the weakest derivative, vinepidine, in B16 melanoma cells. In L-cells, vinblastine completely inhibited growth at 40 nm, whereas vincristine and vindesine caused about 25% inhibition, and vinepidine was inactive. When B16 melanoma cells were treated with drug before being injected into mice, retardation of tumor growth was best achieved with vindesine, one of the weaker of the four derivatives in vitro. The results demonstrate that chemical differences among the Catharanthus derivatives, which affect to small extents the abilities of the derivatives to inhibit microtubule assembly in vitro, result in significant differences in the order and the magnitude of the abilities of the drugs to inhibit cell growth.