In patients with nonresectable cancer quantitated suspensions of cells from their own tumor were transplanted subcutaneously in serial dilutions and observed for growth. In parallel were implants of tumor cells mixed with the patient's own leukocytes. Comparison revealed an inhibition of tumor cell growth by the admixed leukocytes in half of the 41 patients who were suitable for evaluation. In 21 of these patients there was a third parallel series of autotransplants of tumor cells mixed with autologous plasma. The plasma inhibited tumor cell growth in only 7, and only in patients whose leukocytes were also inhibitory. In 9 patients there was a row of transplants consisting of autologous cancer cells mixed with homologous leukocytes from a healthy donor. Results in this group showed no consistent pattern. Similar suspensions of tumor cells, with and without admixed leukocytes, were homotransplanted subcutaneously to patients with advanced cancer. Only 9 of these studies gave evaluatable homograft nodules but 7 of them showed evidence of inhibition of the implanted cancer cells by the leukocytes of the cancer donor. The studies are not sufficient in number to be statistically significant, but they suggest that the leukocytes (? lymphocytes) of cancer patients have a specific inhibitory effect on the growth of the cancer cells of that individual.