Each cell line was incubated with 0.5–200 μM [³H]vinblastine for 4 h. Subsequently, cells were washed and processed for cell-associated radioactivity as described underExperimental Procedures. Data are given as the mean ± S.D. obtained from multiple individual experimental points from two separate experiments. K _m is the concentration that gives one-half of the maximal accumulation of vinblastine in the cells;V _max is the maximal concentration of vinblastine accumulation in the cells.