External ATP causes a remarkable change in the passive permeability of the plasma membrane in several types of transformed cells. When mouse melanoma cells, Clone-M3, were exposed to ATP in Tris-buffered saline, a great increase in the passive permeability was induced within several minutes. Longer exposure of Clone-M3 cells to external ATP led to a decrease in cell viability. Similar results were obtained with Ehrlich ascites cells, but none of these ATP effects were noted in untransformed cells such as NIH 3T3 cells or BALB/c mouse embryonic fibroblasts. The in vitro cytotoxic effects of antitumor agents (5-fluorouracil, adriamycin, mitomycin C and nimustine hydrochloride) against Clone-M3 cells were additively potentiated by treatment with external ATP, which also synergistically enhanced the cytotoxicity of vincristine. However, the effects of these drugs on mouse embryonic fibroblasts were not modulated by ATP. These results suggest that ATP-treatment is a useful means of enhancing a selective toxicity for tumor cells.