Is beta-carotene an antioxidant?

An hypothesis is presented that is opposed to the conventional viewpoint that beta-carotene is an in vivo free-radical scavenger. It is suggested that there are biochemical reasons why beta-carotene, other carotenoids, and especially their metabolites may be harmful to mammalian systems. Finally, the hypothesis that the macular pigment carotenoids, lutein and zeaxanthin, are free-radical scavengers is challenged.