To evaluate the immunomodulatory effects of beta-carotene we performed a randomized, double-blind trial in healthy male cigarette smokers. Lymphocyte subsets in peripheral blood were assessed by using double labeling with monoclonal antibodies before and after 14 wk beta-carotene (20 mg/d; n = 21) or placebo (n = 24) supplements. In addition we measured the ex vivo phytohemagglutinin and concanavalin A induced lymphocyte proliferation in a separate group (23 placebo, 24 beta-carotene). The beta-carotene and placebo groups were comparable on all initial characteristics. During the intervention plasma concentrations of beta-carotene increased 13-fold in the treatment group whereas retinol concentrations remained constant. Beta-carotene had no effect on lymphocyte subpopulations in peripheral blood. After treatment the beta-carotene group showed 12% higher PHA-induced lymphocyte proliferations than the placebo group (P = 0.02). For ConA induced proliferations no significant difference was observed. These results suggest that supplementary beta-carotene can moderately enhance certain aspects of immune response in healthy male cigarette smokers.