A new medium-term in vivo model was tried using pulmonary adenoma induced by benzo(a)pyrene (BP) in newborn mice. Both inbred mice such as C57BL/5J, C57BR/cdJ. A/J mice and non inbred N:GP(S) mice were used. Benzo(a)pyrene was injected in the subscapular region of newborn mice within 24 hours after birth at a dose of 0.5 mg and 1 mg per mouse, respectively. After 9 weeks lung tumor induced in N:GP(S) and A/J mice but in the other mice. The dose showing a 50% tumor incidence was found in N:GP(S) mice to be 0.5 mg of BP but the tumor incidence was very high in A/J mice even at 40 micrograms of BP, the lowest dose in this experiment. To verify the utility of this model, ascorbic acid, carrot, beta carotene, soybean lecithin, spinach, Sesamum indicum, Ganoderma lucidum, caffeine, red ginseng extract, fresh ginseng and 13-cis retinoic acid, some of which are known to have anticarcinogenic activity in various animal models, were tried with this system. Ascorbic acid, soybean lecithin, Ganoderma lucidum, caffeine and red ginseng extract showed inhibition of lung tumor incidence, while fresh ginseng, carrot, beta carotene, spinach and 13-cis retinoic acid did not. This result suggested that the 9-week medium-term model using lung tumor induced by 0.5 mg of BP was useful for the screening of cancer preventive agents.