Granulocyte-macrophage colony-stimulating factor (GM-CSF) was added to a culture of human peripheral blood mononuclear cells in the presence of interleukin 2 (IL-2) in vitro to elucidate its effect on the induction of lymphokine-activated killer (LAK) cells. Viable cell counts of cultured cells and their cytotoxic effects against natural killer (NK) cell-resistant Daudi cells and NK cell-sensitive K562 cells were measured using the trypan blue dye exclusion test and a 51Cr release assay from the tumor cells, respectively. Although GM-CSF alone did not influence either the cytotoxicities or the surface phenotypes of the cultured cells, the viable cell counts were significantly increased by the addition of GM-CSF in the presence of IL-2 (P less than 0.01). These findings indicate that the addition of GM-CSF in the presence of IL-2 during the induction of LAK cells is useful for obtaining a larger number of effector cells which possess substantial cytotoxic activity.