Nonsteroidal anti-inflammatory and other pharmacological agents were tested in the efferent arc of the tuberculin skin reaction in the guinea pig. A low but significant inhibition was produced by the acidic anti-inflammatory agents, including the arylalkanoic acids, the anthranilic acids, indomethacin, and phenylbutazone. Some nonacidic anti-inflammatory agents also were inhibitory; benzydamine caused a high inhibition. Except for methotrexate, the antineoplastic--antimetabolite agents were not potent inhibitors. Inhibition without toxicity was produced by the coumarin anticoagulants, the diuretics, chloroquine, tilorone, and the following immunoregulators: antithymocyte gamma-globulin, cyclophosphamide, and penicillamine. Levamisole, colchicine, heparin, and niridazole were inhibitory but produced lethality either at or above the effective dose.