Technical Abstract: Carotenoids, plant pigments with potent antioxidant activity, are implicated in chronic disease protection. They are absorbed from the diet and transported by plasma lipoproteins. Monocytes, as circulating blood cells, are exposed to carotenoid-rich lipoproteins. Such exposure may lead to enrichment with carotenoids and may affect the functions of monocyte-derived macrophages. This study explored the impact of cellular enrichment in vitro with '-carotene, lycopene, or lutein on monocyte/macrophage function, using U937 cells as a model. Cell proliferation, production of reactive oxygen species, and cell-substrate adhesion were examined. Maximal carotenoid levels in medium supplemented with pre-enriched human serum were 2-8 x 10'6 mol/L; incubation for 1-6 days resulted in 0.2-1.1 nmol carotenoid/mg cell protein (0.25-1 nmol/ 106 cells), about 10-fold less than that reported in normal tissue in vivo but within the range that might be anticipated with dietary supplementation. Beta-carotene, lycopene and lutein markedly inhibited proliferation of U937 cells, to an extent similar to or greater than that observed with phorbol myristic acetate, a known differentiation/activation agent. Lycopene, but not beta-carotene or lutein, caused a significant increase in reactive oxygen species indicating the induction of cell differentiation. Adhesion and LDL oxidation were unaffected. Thus, cellular carotenoids inhibit proliferation, and for lycopene at least, this may involve cell differentiation. The effectiveness of lycopene, a non provitamin A carotenoid, is consistent with a retinoid-independent pathway modulating cell function.