We have investigated the efficacy of graft-versus-leukemia (GVL) effects induced by cells obtained from different syngeneic and allogeneic lymphoid compartments, by comparing the response to cell therapy with syngeneic (BALB/c x C57BL/6)F1 (H-2d/b) (F1) or allogeneic C57BL/6 (H-2b) (B6) lymphocytes in F1 recipients inoculated with B-cell leukemia (BCL1) of BALB/c (H-2d) origin. Eradication of BCL1 was confirmed in vivo by adoptive transfer of 10(5) spleen cells obtained from treated mice into syngeneic BALB/c recipients. Immunotherapy induced by allogeneic but not syngeneic spleen and lymph node lymphocytes was therapeutically more effective than thymocytes and bone marrow cells (BMC). Alloreactive cells could be further activated in vivo with recombinant human interleukin-2 (rIL-2). The GVL effect of allogeneic lymphocytes was cell-dose-dependent; a heavy leukemia load was more efficiently eradicated after three doses than after a single dose of allogeneic spleen cells (100% versus 23% disease-free survival rate of secondary adoptive recipients respectively). The GVL effect induced by allogeneic spleen cells was preserved after ex vivo exposure of cells to 250 cGy, but not 500 cGy or more. Interestingly, GVL was preserved following administration of ex vivo irradiated (500 cGy) spleen cells when rIL-2 was administered in vivo (p < 0.05). Syngeneic effector cells did not induce GVL, regardless of in vitro and in vivo activation with rIL-2. Our data suggest that allogeneic but not syngeneic (in analogy to autologous) cell therapy may be an effective tool to control residual leukemia following high-dose chemo-radiotherapy. The feasibility of augmenting GVL by successive doses of activated allogeneic donor lymphocytes, partly inactivated in vitro by low-dose ionizing irradiation to prevent severe graft-versus-host disease (GVHD), may lead to safer therapeutic approaches that can be used to reduce the incidence of relapse while avoiding the risk of uncontrolled GVHD.