Opiate use in humans is associated with a reduction in the number of circulating T-lymphocytes and in their ability to undergo transformation, probably resulting from opiate binding to T-cell antigens. (See Table 4). Patients also manifest a diffuse hyperglobulinemia, without change in circulating B-cell numbers. In vitro exposure of rodent and human lymphocytes to cannabinoids depresses their transformation, natural killer activity, and interferon production. Similar results occur with in vivo exposure of rodents, but data in man are mixed. Cannabinoids also inhibit primary and secondary antibody responses in rodents. Data regarding the effects of CNS stimulants are too scanty to allow comment. Barbiturate anesthesia in man and animals produces a short-lived depression of lymphocyte transformation, NK activity, and ADCC; no studies of the effects of chronic exposure have been performed. Amyl nitrite has not been proven to possess any immunosuppressive activity.