Background: β-Carotene has been shown to enhance immune functions in humans. Whether vegetables rich in carotenoids, such as β-carotene or lycopene, modulate immune functions in healthy humans is presently not known. The objective of this study was to investigate the effects of a low-carotenoid diet supplemented with either tomato (providing high amounts of lycopene) or carrot juice (providing high amounts of α- and β-carotene) on immune functions in healthy men. Method: In a blinded, randomized, cross-over study, male subjects on a low-carotenoid diet consumed 330 ml/day of either tomato juice (37.0 mg/day lycopene) or carrot juice (27.1 mg/day β-carotene and 13.1 mg/day α-carotene) for 2 weeks with a 2-week depletion period after juice intervention. Immune status was assessed by measuring lytic activity of natural killer (NK) cells, secretion of cytokines (IL-2, IL-4, TNFα), and proliferation by activated peripheral blood mononuclear cells. Results: Juice consumption resulted in relatively fast responses in plasma carotenoid concentrations (p < 0.0002) which were not accompanied by concomitant changes in immune functions. For IL-2, NK cell cytotoxicity, and lymphocyte proliferation, maximum responses were observed during depletion periods. The highest production rate was measured only for TNFα at the end of the first intervention period. Juice intervention did not modulate the secretion of IL-4. Conclusions: Increased plasma carotenoid concentrations after vegetable juice consumption are accompanied by a time-delayed modulation of immune functions in healthy men consuming a low-carotenoid diet.