BACKGROUND: L-Arginine has been described as a potential immunostimulant in vitro and in vivo. Excessive arginine, however, may be counterproductive. Data support the concept of minimal arginine requirements for normal lymphocyte proliferation, but the results of supplementation with pharmacologic doses of arginine have been contradictory. We hypothesized that excessive arginine supplementation might result in a blunting of normal immune responses of human lymphocytes in vitro. MATERIALS AND METHODS: Peripheral blood mononuclear and T-cells were isolated from normal human donors. Cells were cultured in complete media with various concentrations of L-arginine, L-ornithine, and glycine. Lymphocytes were then stimulated with PHA or alloantigens, and proliferation was determined by measuring [3H]thymidine incorporation. RESULTS: Lymphocyte proliferation was inhibited by L-arginine at pharmacologic doses. The effects were completely reversible. This inhibition could not be prevented by lymphocyte stimulation with IL-2. Lymphocyte proliferation was more sensitive to inhibition by lower doses of arginine when alloantigens from irradiated fresh tumor cells or allogeneic lymphocytes were the stimuli. Finally, lymphocytes showed variable sensitivity to inhibition of proliferation in response to mitogen when treated with L-ornithine (little to no effect) or L-arginine (consistent inhibition at high doses). Pharmacologic doses of L-arginine result in reversible inhibition of normal lymphocyte proliferation in response to both mitogen and alloantigen. This inhibition could not be blocked by interleukin-2. CONCLUSIONS: We conclude that caution should be exercised when recommending aggressive L-arginine supplementation as a possible method to reverse clinical immunosuppression caused by cancer, malnutrition, or trauma.