This study compares the toxic effects of the carotenoids, beta-carotene and canthaxanthin, and alpha-tocopherol (vitamin E) on human tumor cells and their normal counterparts in vitro. Seven different malignant cell lines were examined: oral carcinoma (two cell lines), breast (two cell lines), lung carcinoma (two cell lines), and malignant melanoma. The in vitro cell culture assays showed a consistent morphologic change in the affected tumor cells following treatment with carotenoid or vitamin E. A rounding of the tumor cells and eventual lifting off the tissue culture plate were observed. These changes were apparent after 1 to 5 hours of treatment depending on the tumor cell line. Associated with these observable cellular changes were quantitative reductions in proliferation (3H-thymidine proliferation) and succinic dehydrogenase activity (MTT assay). In addition, there was a noticeable change in protein expression, with an increased expression of a 70-kD protein following treatment with beta-carotene. This protein was associated with tumor cells showing a decrease in proliferation (oral carcinoma, malignant melanoma) but not with normal keratinocytes or melanocytes. These studies substantiate a selective cytotoxic effect on human tumor cell growth by carotenoids and alpha-tocopherol in vitro, and may provide an explanation of the therapeutic activity of these agents and their possible use in the treatment of premalignancy or early oral carcinoma.