Lung cancer is one of the most common cancers in the world, and the incidence of lung cancer is increasing. Therefore, it is particularly important to detect carcinogenic or tumor promoting substances of lung carcinogenesis in our environment, so that such harmful chemicals can be eliminated from our environment. Furthermore, detection of chemopreventive agents of lung carcinogenesis is also important to reduce our risk of lung cancer. It is necessary to establish reliable in vivo animal models of lung carcinogenesis for that purpose. The A/J mouse is a mouse strain sensitive to lung carcinogens, and also develops spontaneous lung tumors without any chemical treatment. In our department, we have demonstrated that a treatment of 4-(methylnitrosamino)-1-(3-pyridyle)-1-butanone (NNK), a tobacco specific nitrosamine, or 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (MeIQx), a heterocyclic amine, induced lung tumors in the female A/J mouse in 16 and 32 weeks. The lung tumors developed in the A/J mouse are histopathologically classified as adenocarcinomas, adenomas, and alveolar cell hyperplasias. Some of these types of lung cancer are similar to those of human lung cancer. We also investigated the chemopreventive effects of bovine LF (bLF) on different phases of NNK-induced lung tumorigenesis in A/J mice. The A/J mouse is very useful mouse strain as a reliable in vivo model, which can be used for detecting chemopreventive agents of lung carcinogenesis.