Consumption of carotenoids has frequently been inversely correlated with cancer incidence. In this report we used the 7,12-dimethyl-benz[a]anthracene (DMBA)-induced rat mammary tumor model to compare the effect of lycopene-enriched tomato oleoresin on the initiation and progression of these tumors with that of beta-carotene. Rats were injected i.p. with lycopene-enriched tomato oleoresin or beta-carotene (10 mg/kg, twice per week) for 2 weeks prior to tumor induction by DMBA and for an additional 16 weeks after carcinogen administration. HPLC analysis of carotenoids extracted from several tissues showed that both carotenoids were absorbed into blood, liver, mammary gland, and mammary tumors. The tomato oleoresin-treated rats developed significantly fewer tumors, and the tumor area was smaller than that of the unsupplemented rats. Rats receiving beta-carotene showed no protection against the development of mammary cancer.