Carotenoids are shown to inhibit the initiation phase of carcinogenesis due to their antioxidant activity. But neither antioxidant nor pro-vitamin properties of these pigments can explain their antitumor action in many cases. Studying this problem another property of carotenoids must be accounted: their ability to modify biomembranes reinforcing lipid bilayer and decreasing its fluidity. High level of membrane fluidity is characteristic of the tumor cells and they are very sensitive to its decrease. Suppressing of membrane fluidity is the mechanism for anticancer action of tamoxifen-related compounds, and there are strong reasons to suggest the same mechanism for carotenoids.