Benz(e)aceanthrylene and benz(1)aceanthrylene, cyclopenta-fused derivatives of benz(1)anthracene, have been reported to be active bacterial cell and mammalian cell gene mutagens. In this study they were evaluated as skin tumor initiators in both male and female SENCAR mice. Both PAH induced papilloma formation in the range of 50-1000 nmol/mouse. Benz(1)aceanthrylene was the most active, being approximately 4 times as activeas benzo(a)pyrene while benz(e)aceanthrylene had activity approximately equivalent to benzo(a)pyrene. These results are in contrast to those reported for the air pollutant, cyclopenta(cd)pyrene, another cyclopenta-fused PAH, which is a weak mouse skin tumor initiator. The authors postulate that these cyclopenta-PAH are formed by pyrosynthetic routes similar to other environmental cyclopenta-PAH and that they may be of importance as contributors to air pollution carcinogenesis. (Copyright (c) 1984 Elsevier Scientific Publishers Ireland Ltd.)