In 2 hamster cell lines that differed 100-fold in their vinblastine sensitivity, dissolution of the mitotic spindle by vinblastine in living cells correlated with cytotoxicity from vinblastine expressed as suppression of colony formation. The effect on the spindle apparatus occurred in 30 s or less and provides a rapid assay for determining the cytotoxic effects of the Vinca alkaloids, and the potential for quantitative assay of solutions of Vinca derivatives.