A previous study demonstrated that latent tumor cells induced in mouse skin by the carcinogenic hydrocarbon 7,11- dimethylbenz(a)anthracene were irreversibly changed for up to 43 weeks. This was proven by treatment after this interval with the promoting agent, croton oil. Repetition of this study has shown that some of the changed cells may have reverted to the normal state after a longer period. The repetition of this study is not conclusive, and additional large scale study is necessary to deny or confirm this conclusion. The concept that a small dose of a carcinogen induces irreversible changes is one of the bases for the contention that any dose of a carcinogen may be hazardous and this original experiment has long been used in support. In addition, these studies are of practical interest in predicting the reduction in lung cancer rate among ex-smokers.