Vinblastine (VLB) and vincristine (VCR) have been compared with respect to their inhibitory effects on the growth of cultured Nb2 node rat lymphoma cells and their binding and release by these cells. When examined over a wide concentration range, the two alkaloids were found to be almost equally effective in inhibiting culture growth, providing they were continuously present in the incubation medium. A striking difference was found, however, when the cells were incubated for short periods with the alkaloids and then restored to alkaloid-free medium for further incubation. Cells treated with VCR (5 X 10(-9) g/ml) for 3 h and washed free of extracellular alkaloid failed to resume normal proliferation during a subsequent 48 h incubation period in alkaloid-free medium. In contrast, cells treated with VLB at a higher concentration (5 X 10(-8) g/ml) and for a longer time (7 h), rapidly resumed growth in alkaloid-free medium. Binding studies, using (3H)VLB and (3H)VCR, showed that when cells were incubated continuously with the alkaloids at a given concentration, they bound equal amounts of the drugs. When labelled cells were resuspended in alkaloid-free medium, VLB was released very readily by the cells, whereas VCR was tenaciously retained. The findings indicate that the growth-inhibitory effects of VLB and VCR are related to the amount of alkaloid associated with the cells and that the long-lasting effects of VCR, relative to VLB, are very likely due to the retention of VCR by the cells when the extracellular alkaloid levels decline. The study also suggests that, in the treatment of malignancies, VLB could be more effective if its extracellular levels were maintained, e.g. by continuous administration.