Therapeutic efficacy of allogeneic cells correlates with the intensity of the lymphodepleting preparatory regimen. C57BL/6 mice bearing B16 tumors were irradiated with 5, 7, 9, or 11 Gy TBI. For each irradiation dose, mice were left untreated as a control (NT) or injected on day 0 with 10⁶ allogeneic pmel-1^b/d cells, vaccinia virus encoding hgp100, and exogenous rhIL-2 (PVI). All groups received syngeneic BMT with 10⁶ unsorted bone marrow cells the day after the transfer of the effector cells. Statistical results are as follows: 5 Gy NT versus 5 Gy PVI, not significant; 11 Gy NT versus 11 Gy PVI, P < .02; 5 Gy PVI versus 7 Gy PVI, P < .04; 7 Gy PVI versus 9 Gy PVI, P < .03. There was no statistical difference between mice receiving 9 Gy versus 11 Gy plus PVI. Results of tumor area are the mean of measurements of at least 5 mice per group (± SEM). Data are representative of 2 independent experiments.