We studied the in vitro effects of lymphokine-activated killer (LAK) cells from the peripheral blood of chronic myeloid leukemia (CML) patients after allogeneic and syngeneic bone marrow transplantation (BMT). LAK cells were generated by incubating peripheral blood mononuclear cells from patients post-BMT with recombinant interleukin-2 (IL-2) (500 U/mL) in 10% AB serum for 7 days. They were phenotyped and tested for activity in a standard 4-hour 51Cr release assay (n = 37) and in a CFU-GM assay (n = 24). We found that the LAK cells were mainly activated natural killer cells, but some were CD3+ T cells. In the 51Cr release assay LAK cells from 20 of 33 (61%) allogeneic and 2 of 4 syngeneic recipients killed recipient CML cells and in 22 of 37 (60%) cases also killed the HLA disparate CML cells. In the CFU-GM assay the LAK cells incubated together with the CML cells in liquid culture before plating inhibited (P less than .05) colony growth in 16 of 22 allogeneic and 2 of 2 syngeneic recipients. Cell-cell contact was necessary for optimal effect. There was little or no inhibition of proliferation of donor marrow CFU-GM. This in vitro graft-versus- leukemia (GVL) effect could also be demonstrated after LAK effectors were depleted of CD3+ T cells. It was inducible in recipients of both T cell-depleted and T cell-replete donor marrow and in recipients with or without graft-versus-host disease. These results suggest that a major histocompatibility complex-unrestricted GVL effect is inducible following allogeneic and syngeneic BMT. The use of IL-2/LAK cells after BMT could reduce the risk of relapse.