Immunotherapy with recombinant human interleukin-2 (IL-2) and allogeneic spleen cells has led to significant antitumor effects in B-cell leukemia- (BCL1) bearing mice following transplantation with T-lymphocyte-depleted allogeneic bone marrow cells. Graft versus leukemia (GVL) effects were studied in a model mimicking minimal residual disease following bone marrow transplantation (BMT). Lethally irradiated (BALB/c X C57BL/6)F1 recipients were reconstituted with 20 X 105T-lymphocyte-depleted C57BL/6 bone marrow cells mixed with 104to 106BCL1 cells followed by administration of sequential increments of allogeneic C57BL/6 spleen cells; 106cells on Day +1, 107cells on Day +5, and 5 X 107cells on Day +9, with or without concomitant IL-2 treatment (intraperitoneal injections of 20,000 U twice daily for 3 days) together with each spleen cell administration. All mice receiving 10t-106BCL1 cells developed marked splenomegaly by Day +21 and all adoptive recipients of 10sspleen cells obtained from these mice developed leukemia within 21-36 days. Treatment of mice which received 104BCL1 cells by either three courses of low dose IL-2 or three increments of allogeneic spleen cells alone and certainly by a combination of both resulted in normalization of splenomegaly on Day +21, but only adoptive recipients of 105spleen cells obtained from mice treated by both allogeneic spleen cells and IL-2 (10/10) or allogeneic spleen cells alone (8/10) were disease free (>100 days). Mice inoculated with 105BCL1 cells developed mild splenomegaly on Day +21 after R2 treatment alone, but showed no clinical evidence of disease following administration of allogeneic spleen cells or both allogeneic spleen cells and IL-2. Following adoptive transfer of 10sspleen cells obtained from each treated group no leukemia (>100 days) was evident in recipients of spleen cells obtained from mice treated with both, allogeneic spleen cells and IL-2 (10/10) whereas a partial effect was observed in mice treated by allogeneic spleen cells only (4/10). Mice inoculated with a high dose ofBCLl cells (106) showed some delay in onset of splenomegaly, but no curative antileukemic effects could be observed even following a synergistic combination of IL-2 and allogeneic spleen cells. Our data suggest that immunocompetent allogeneic lymphocytes may play an important role against leukemic relapse and thus cell therapy may be used therapeutically to treat minimal residual disease after BMT even following initial reconstitution with T-cell-depleted bone marrow cells. Moreover, GVL effected mediated by allogeneic lymphocytes may be further augmented by concomitant administration of suboptimal doses of IL-2. A similar approach may prove beneficial in conjunction with autologous and allogeneic BMT in humans as long as graft versus host disease can be prevented or controlled.