We have investigated the subpopulations of T cells necessary to mediate the cure of established tumors in two models of successful adoptive immunotherapy. In C57BL/6 mice bearing palpable and disseminated FBL-3 lymphoma, both Lyt-1+ and Lyt-2+ cells played a major role in mediating the regression and permanent cure of mice, whereas in BALB/c mice bearing the Meth A sarcoma the adoptive transfer of Lyt-1+2+ cells played a major role in mediating the regression of tumors and the curing of disease. Identical experiments performed in hybrid (BALB/c X C57BL/6) mice yielded similar results, further supporting our initial observation and indicating that in these two adoptive transfer model systems it is the tumor and not the variable expression of Lyt antigens by the host that determines which T cell subpopulation is required to cure mice of tumors.