The carcinogenic potential of 1-methylnaphthalene (1-MN), a compound which exists widely in the environment, was investigated in B6C3F1 mice. Groups of 50 male and 50 female mice were given diets containing 0, 0.075, or 0.15% 1-MN for 81 weeks. Both treatment groups developed pulmonary alveolar proteinosis at high incidence, with 46.0 and 34.7% of females and 46.0 and 38.0% of males, respectively, being affected. Total lipid and phospholipid levels in sera and monocytes in peripheral blood were also significantly increased in 1-MN-treated female and male mice in contrast with control values. The incidences of bronchiolar/alveolar adenomas in the lungs of male mice given both 0.075 or 0.15% 1-MN were 26.0 and 24.0%, respectively, in both cases significantly increased in contrast with the 4.1% observed for control males. However, neither dose dependence nor significant difference in the incidences of bronchiolar/alveolar carcinomas between 1-MN-treated and control male mice was observed. The incidences of other tumors also were similar in both 1-MN-treated and control groups. The results of the present experiment thus suggested a possible weak carcinogenic potential of 1-MN to the lung of male but not female B6C3F1 mice.