A human intervention study was conducted to determine the effect of the consumption of carotenoid-rich vegetables on the immune system. Subjects, (twenty-three men), who were non-smokers, were not restricted in their daily diet, except that they had to abstain from fruit and vegetables high in carotenoids throughout the whole study period. The study was divided into four periods, each lasting 2 weeks: weeks 1-2: low-carotenoid period; throughout weeks 3-8: daily consumption of 330 ml tomato juice (40 mg lycopene/d, 1.5 mg beta-carotene/d) (weeks 3-4), 330 ml carrot juice (21.6 mg beta-carotene/d, 15.7 mg alpha-carotene/d, 0.5 mg lutein/d) (weeks 5-6), 10 g dried spinach powder (11.3 mg lutein/d, 3.1 mg beta-carotene/d) (weeks 7-8). Blood was collected weekly from subjects after a 12 h fast. T-lymphocyte functions were assessed by measuring proliferation and secretion of immunoreactive cytokines. The consumption of a low-carotenoid diet resulted in a significantly reduced proliferation of peripheral blood mononuclear cells (PBMC) cultured with concanavalin A. After 2 weeks of tomato juice consumption and until the end of the intervention period lymphocyte proliferation was not significantly changed compared with proliferation at the end of the depletion period. Secretion of cytokines by T-helper-1-like lymphocytes (interleukin (IL)-2) and by T-helper-2-like lymphocytes (IL-4) was influenced by the dietary intervention. IL-2 and IL-4 secretion values were significantly suppressed after the low-carotenoid diet (P < 0.001 and P < 0.05 respectively compared with baseline). Tomato juice consumption significantly enhanced IL-2 (P < 0.001) and IL-4 secretion (P < 0.05) compared with the end of depletion period. After carrot juice and spinach powder consumption the cytokine secretion capacity of PBMC was not significantly different from that at the end of the depletion period. In conclusion, the results of the present study indicate that a low-carotenoid diet reduces T-lymphocyte functions and addition of tomato juice restores these functions. This modulation could not be explained by changes in the plasma carotenoid concentrations. The active constituents in tomato juice as well as the biological significance of this immunomodulation remain to be determined.