The purpose of this study is to examine the carotenoid effects on lung tumori-genesis induced by intratracheal instillation of diesel exhaust particles (DEP) into mice weekly for 20 wk. It was suggested that active oxygen radicals might play an important role in DEP-induced lung tumorigenesis. Mice were divided to 4 groups of diet containing 0.02% of palm oil carotene, 0.02% of β-carotene, or no carotenoid with or without DEP. The BF group (4% fat) and the HF group (16% fat) were prepared for each diet group. The experi-mental period was 12 mo. By the administration of palm oil carotene, neither adenocarci-noma nor adenoma was found in the BF group. In the HF group with palm oil carotene, no adenocarcinoma was observed, and adenoma was reduced. Adenoma in the HF group was not greatly reduced by β-carotene, but rather increased in the BF group. No adenocarci-noma was found in either the BF or the HF groups with β-carotene. The 8-hydroxy-deoxyguanosine/deoxyguanosine ratio in palm carotene groups was lower than in the other groups, while that in β-carotene groups was not. From these results, palm oil carotene was suggested to prevent lung tumorigenesis by its protective effect on DNA from active oxygen. Beta-carotene was supposed to have different effects from palm oil carotene on lung tumori-genesis. Besides the chemopreventive effect, the growth of mice was inhibited by the admin-istration of palm oil carotene. Further studies are necessary to elucidate the mechanisms of carotenoid effects.