Delta-9-tetrahydrocannabinol (THC), the major psychoactive component of marijuana, was tested for its ability to modulate human natural killer (NK) cell function. THC was toxic for peripheral blood lymphocytes at 20 μg/ml but not at 10 μg/ml or less. This component of marijuana also was inhibitory for NK activity against K562, a human tumor cell line at concentrations down to 5 μg/ml when pre-incubated with the effector cells. Suppression of NK function was dependent upon the concentration of THC and the length of time of pre-incubation but was independent of the ratio of effector to target cells. Prostaglandins were not involved in suppression of NK activity.