Male Wistar Kyoto rats were fed diets containing either 2 g/kg (0.2%) beta-carotene, canthaxanthin or basal diet for up to 66 wk. Plasma and tissues were analyzed for vitamin A, vitamin E, beta-carotene or canthaxanthin levels. In vitro immune responses of splenocytes to T- and B-lymphocyte mitogens were determined. T- and B-lymphocyte responses were consistently enhanced in the groups fed beta-carotene or canthaxanthin. Since canthaxanthin cannot be converted to vitamin A, the immunoenhancement seen in these experiments is attributed to a carotenoid effect.