Human malignant melanoma cell lines established in tissue culture were successfully transplanted sc into BALB/c nude mice. The growth rate of the resulting tumors was significantly suppressed when lymphocytes from the patient in whom the tumor arose were injected iv into BALB/c nude mice at the same time as sc tumor transplantation, but inoculation with lymphocytes from a person without a tumor was ineffective. Cell separation identified T-lymphocytes as the active subpopulation. Growth of the tumors was also significantly suppressed by reconstitution of the mice with normal BALB/c lymphocytes; lymphocytes from BALB/c mice previously immunized against the melanoma line were not more effective than nonimmune lymphocytes in preventing tumor growth. Sera from normal BALB/c mice or BALB/c mice immunized against a human melanoma cell line effectively suppressed growth of that cell line in BALB/c nude mice if given at the time of tumor transplant. The results suggested that, whereas murine lymphocytes reconstitute the ability of nude mice to react to xenogeneic antigens on the human tumor, human lymphocytes showed greater specificity to autologous human melanoma-associated antigens.