Bladder cancer was induced in male B6D2F₁ strain mice by the administration of N-butyl-N-(4-hydroxybutyl)nitrosamine. Mice supplemented with β-carotene for 5 weeks before receiving the carcinogen and maintained on β-carotene for an additional 26 weeks developed significantly fewer tumors than did unsupplemented mice. Mice receiving canthaxanthin for the same time period showed no protection against the development of bladder cancer. effect of this molecule must be an intrinsic property of carotenoids.