Accumulation of vinblastine has been studied in different types of cancer cells. We measured the intracellular concentration of vinblastine in MO4 cells and compared it to its extracellular concentration in culture medium. We observed that the ratio of accumulation, expressed as the ratio of intracellular over extracellular vinblastine, was dose-dependent. An initial accumulation level was reached within 30 min and a second dose-induced level after 48 hours. This remained constant for another 5 days. Intracellular accumulation was also seen for vincristine and vindesine but here the phenomenon was less dose-dependent and the maximum level was reached after 6 hours. The accumulation ratio of a single vinca alkaloid in MO4 cells is not different from combined simultaneous exposure with 2 or 3 of the drugs. Energy blocking by sodium azide resulted in an inhibition of the supplementary but not the initial level of vinblastine accumulation. After washing out the medium for vinblastine the intracellular concentration remained about 60 micrograms/ml for 1 microgram/ml vinblastine added to the medium. Based on these results, it is concluded that the accumulation of vinblastine is only partially based on an energy mediated mechanism, in which binding of the drug to at least two types of sites may play an important role.