Three hydrocarbons previously reported to be nontumorigenic initiated tumor formation in mouse skin. In addition, chrysene, benz[a]anthracene, dibenz[a,c]anthracene, and benzo[e]pyrene produced multiple papillomas when used as initiators in modest doses. Pyrene, anthracene, and anthanthrene showed borderline initiating activity. These findings generally agree with previously published calculations of the chemical reactivity of these and other unsubstituted aromatic hydrocarbons, which postulated that substitutions at different positions in the hydrocarbons represent competing processes of activation for carcinogenesis and detoxication.