The ability of lysergic acid diethylamide (LSD) to alter immune function after direct in vitro exposure was examined. It was demonstrated that LSD is able to suppress the proliferation of B-lymphocytes; the production of the cytokines IL-2, IL-4, and IL-6; and the induction of cytotoxic T-lymphocytes at a concentration of 100 microM. In vitro exposure to LSD had differential effects on natural killer (NK) cell activity, with significant enhancement of both basal and IL-2-augmented NK cell function at concentrations between 0.0001 and 0.1 microM, and suppression of NK response at 100 microM. These results demonstrate that LSD may have a direct effect on components of the immune system at concentrations that may be reached upon human exposure.