Clinical experience with three vinca alkaloids currently in use as antineoplastic agents has shown a difference in the degree of peripheral neurotoxicity manifested by these compounds: vincristine greater than vindesine greater than vinblastine. This phenomenon may reflect differences in pharmacokinetics and/or the differential response of the nerve tissue itself. Differences in pharmacokinetics can be avoided by studying the direct effects of the vinca alkaloids on primary cultures of neuronal and glial cells. Vincristine at a dose as low as 0.004 microgram/ml affects the cells with processes in cultures of dissociated newborn rat midbrain. In 3-day-old cultures, after 24 h of drug treatment there is a loss of processes and swelling of the cell body. We have used this observation as the basis for a quantitative assay of the toxicity of a series of vinca compounds, and have found that for a dose range of 0.1--0.004 microgram/ml the relative toxicity of vincristine, vinblastine, and vindesine in this system correlates with their relative clinical neurotoxicity. Validation of the predictive elements of this system awaits clinical experience with novel vinca compounds.