Limiting numbers of peripheral-blood mononuclear cells (PBMC) from melanoma patients were stimulated with irradiated autologous tumor cells in the presence of interleukins-2 and -4 and in the absence of feeder cells. The responder cells were restimulated every week. After 2 to 4 weeks, the microcultures were tested for their lytic activity against the autologous tumor cells. Significant lysis of the tumor cells was observed with a fraction of these microcultures, whereas no lysis was observed with control microcultures seeded without stimulator melanoma cells. Because our aim was to measure the precursor frequency of CTL showing specificity for the tumor, and not that of NK-like effectors that were also capable of lysing the melanoma cells, we used cold-target inhibition with an excess of NK target K562 to inhibit the NK-like activity. Microcultures whose lysis on the tumor cells was not abolished by K562 competition were observed. The specificity of these CTL clones was confirmed by the absence of lytic activity on autologous T-cell blasts. The numbers of microcultures with anti-tumor CTL activity fitted the zero-order of the Poisson distribution equation, indicating that they resulted from the activity of single T-cell clones. The frequency of anti-tumor CTL precursor cells (CTL-P) of 7 melanoma patients ranged from 1/900 to 1/33,000. Frequencies of anti-tumoral CTL-P were higher and NK-like effectors were less frequent when sorted CD8+ T lymphocytes were used as responder cells.