Incubation of human lymphocytes with recombinant interleukin-2 (rIL-2) results in the generation of lymphokine-activated killer (LAK) cells capable of lysing a wide variety of tumor cells. The present study was undertaken to examine the effect of recombinant gamma interferon (rIFN-gamma) on LAK cell cytotoxicity generated from different peripheral blood mononuclear cell (PBMC) subpopulations. When unseparated PBMC were stimulated by rIL-2 and rIFN-gamma, the latter induced a transient enhancement after 2 days followed by a suppression of LAK cell cytotoxicity at day 6. Enhancement of LAK cell cytotoxicity was moderate and inconstant, whereas the inhibition was strong and observed with all the donors tested. This suppression was not associated with a decrease in the [3H]thymidine uptake. PBMC depleted of adherent cells were more sensitive to the stimulation by rIL-2 and the induced cytotoxicity was not modified by rIFN-gamma. Monocyte-enriched plastic-adherent cells, when incubated with rIL-2 and rIFN-gamma, became cytotoxic after 2-3 days of culture and inhibited LAK cell activity after 5-6 days. Collectively, our results suggest that rIFN-gamma affects LAK cell cytotoxicity through the activation of plastic-adherent, monocyte-rich, cells which modulate natural killer cells, first in a positive, then in a negative way.