Epidemiological studies suggest that beta-carotene is able to modulate the risk of cancer. A number of in vitro studies reported that beta-carotene inhibits the growth of cancer cells; however, so far little is known about the molecular mechanisms of the antiproliferative effect of beta-carotene. Here we have investigated the effects of two beta-carotene preparations, (i) beta-carotene dissolved in tetrahydrofuran (final concentration in cell culture medium: 0.5%) and (ii) beta-carotene incorporated in a water dispersible bead form, on cultured human colon carcinoma cells HT29. The treatment of cells with beta-carotene up to 30 microM for 72 h led to a significant increase in the cellular beta-carotene concentration and formation of retinol. Beta-Carotene showed only low cytotoxicity for confluent cells tested up to 30 microM, but at dietary relevant concentrations for the intestinal tract (10, 30 microM) beta-carotene was strongly cytotoxic for growing cells and induced apoptosis in HT29 cells as assessed by the Annexin-V assay (the maximal effect was observed 15 h after treatment with beta-carotene). Exposure of cells to retinol at concentrations yielding cellular retinol levels similar to those observed by beta-carotene treatment had no antiproliferative or cytotoxic effect. Furthermore, beta-carotene did not affect the activation of the extracellular signal-regulated kinases (ERK1 and ERK2) that are essential for cellular growth. In summary, beta-carotene can inhibit growth of human colon carcinoma cells in vitro by induction of apoptosis in proliferating cells.