Suppressor T lymphocytes possess histamine2 (H2) receptors and contribute significantly to the function of the immune system. Experimentally, cimetidine, an H2-receptor antagonist, has been shown to enhance a variety of immunologic functions both in vivo and in vitro because of its inhibitory effects on suppressor-cell function. Successful tumor immunotherapy, as well as some protection from infection, has been reported in experimental animals. Patients receiving cimetidine have been shown to exhibit enhanced cell-mediated immunity as evaluated by increased response to skin-test antigens, restoration of sensitivity following development of acquired tolerance, and increased responses of lymphocytes to mitogen stimulation. Preliminary reports also indicate that cimetidine may offer therapeutic benefits for patients with Varicella zoster and Herpes simplex infections, as well as those suffering from mucocutaneous candidiasis and common variable hypogammaglobulinemia. These immunoregulatory effects are dose-related but are not always consistent. Because of its inhibitory effect on suppressor function, cimetidine treatment may be deleterious in patients with organ transplant and autoimmune disorders. Cimetidine should be used as an immunomodulator on an experimental basis only.