Epidemiological studies suggest that carotenoids may play a role in human breast carcinogenesis. To identify an anticarcinogenic mechanism, a laboratory model for examination of biologic effects is required. Efficacy of tetrahydrofuran (THF) for delivery of beta-carotene to a human mammary epithelial cell line has not been reported, and biologic effects of carotenoids on normal mammary epithelial cells or mammary epithelial cell lines have not been described. In these studies, we examined MCF-10A cells treated with 0.04%, 0.10%, and 0.35% THF (vol/vol) for morphological signs of toxicity and determined effects of THF on cell proliferation over a seven-day period. Cells treated with THF demonstrated a reduction in mean number of cells per dish (p < 0.05) but still underwent a 3.2- to 4.0-fold increase in cell number over the seven days. MCF-10A cells were also treated with a 7 mumol/l solution of beta-carotene and examined for morphological changes and effects on cell growth. Exposure to this concentration of carotenoid did not significantly affect proliferation but did induce the formation of cytoplasmic vacuoles similar to those seen in differentiating mammary epithelial cells. High-performance liquid chromatography analysis revealed a beta-carotene concentration of 0.004 nmol/10(6) cells in the treatment group. The effects of beta-carotene and the non-provitamin A carotenoid canthaxanthin were also examined in the in vitro cultures of primary human mammary epithelial cells obtained from reduction mammoplasties of two individuals. Exposure to these carotenoids induced morphological changes consistent with cellular differentiation and had a dramatic effect on the proliferative life span of these cells. Thus carotenoids may directly affect the proliferative capacity and differentiation of mammary epithelial cells, which may be among the chemoprotective activities of these compounds.