Previous reports have demonstrated granulocyte-macrophage colony-stimulating factor (GM-CSF)-mediated enhancement of lymphokine activated killer (LAK) cell function. Based on these studies we have developed a model of LAK cell generation from peripheral blood stem cells (PBSC) from cancer patients by in vitro incubation with low-dose interleukin-2 (IL-2) + GM-CSF. PBSC from seven patients were incubated for 48 h at 37 degrees C in serum-free culture medium supplemented with IL-2 at increasing concentrations (10, 100 or 1000 IU/ml) in the presence or absence of 10 IU/ml GM-CSF. LAK activity generated in cultures with 10 IU/ml IL-2 + GM-CSF was significantly higher than that generated by 10 IU/ml IL-2 and did not differ from LAK generation at optimal concentrations of IL-2 (100 and 1000 IU/ml). PBSC from five additional patients were incubated with low-dose IL-2 + GM-CSF after sequential depletion of the CD4+ and CD8+ T cell subsets. LAK activity was significantly reduced by depletion of both CD4+ and CD8+ T cells and almost completely abolished after depletion of both subsets, suggesting that T cells and not NK cells are the main LAK precursors in this model. Six patients have received two courses of LAK cells generated in vitro by low-dose IL-2 + GM-CSF on day +1 and +8 after PBSC transplant in combination with GM-CSF and IL-2 administration in vivo. The mean LAK activity in peripheral blood of these patients dramatically increased immediately after transplant from a mean of 10% to 43.2% on day +2 and remained increased during the period studied. These results are encouraging and suggest that the administration of in vitro generated LAK cells early after transplant may have a role in the control of minimal residual disease.