The effect of benzo (a) pyrene on the growth in culture of 5 mouse epithelial cell strains was examined. These epithelial cells are highly sensitive to the cytotoxic action of benzo (a)-pyrene. In addition, the activity of the benzol (a) pyrene-metabolizing system, aryl hydrocarbon hydroxylase, is low but highly iducible by the carcinogen. As the sensitivity of a cell strain to the cytotoxic action of benjo (a) pyrene decreased, the inudcibility of the hydroxylase also decreased,. However, a strong correlation could not be found between cytotoxicity and the level of uninduced or induced hydroxylase when the values from different cell strains were compared. These experiments suggest that thehydroxylase is important in determining the sensitivity of epithelial cells to the cytotoxic action of benzo (a) pyrene, but other factors may also modulate this sensitivity.