Fifteen cyclopenta[a]phenanthrenes and two chrysenes, previously tested for their ability to induce skin tumours in mice of the Theiler's Original (TO) strain, were retested in Sencar mice by the initiation/promotion regime. The five compounds found to be strong carcinogens in TO mice were approximately equally active in Sencar mice with latent periods not noticably shorter, as had been anticipated from the results of others with polycylic aromatic hydrocarbons such as benzo[a]pyrene. Ten compounds inactive in TO mice also failed to induce tumours in Sencar mice. Of the remaining two compounds, both ethyl derivatives, one was apparently more active in Sencar mice, whilst the other showed the opposite behaviour. Possible reasons for these results are tentatively discussed.