Summary The tumor initiating activity on mouse skin of benzo[ a]pyrene (BaP), 7,10-dimethylBaP, and 10-methylBaP was determined. Each compound was tested at initiating closes of 50 μg and 10 μg with promotion by application 3 times weekly of 2.5 μg tetradecanoylphorbol acetate. BaP induced tumors in 40% (100 μg) and 25% (50 μg) of the animals. No tumors were observed in either group treated with 7,10-dimethylBaP. In the groups treated with 10-methylBaP, the incidence of tumor bearing animals was 20% at both doses. These results and the results of previous studies on other methylated BaP derivatives suggest that the mechanism of activation of these compounds is similar to that observed for the parent hydrocarbon and probably involves formation of an angular ring diol-epoxide or epoxide.