Cyclopenta-polycyclic aromatic hydrocarbons are a unique class of environmental PAH which contain an unsaturated five-membered ring available for metabolic alteration and metabolic activation. Previous studies with four isomeric cyclopenta-derivatives of benz(a)anthracene: benz(e)aceanthrylene, benz(j)aceanthrylene, benz(l)aceanthrylene, and benz(k)acephenanthrylene indicated that these PAH were mutagenic to Salmonella typhimurium and Chinese hamster V79 cells. In the study, these four isomeric cyclopenta-derivatives of benz(a)anthracene were examined for their ability to morphologically transform C3H10T1/2CL8 mouse embryo fibroblasts. All of the PAH studied except benz(k)acephenanthrylene transformed C3H10T1/2CL8 cells to both type II and type III foci in a concentration-dependent fashion. Benz(j)aceanthrylene was the most active, equivalent in activity to benzo(a)pyrene on a molar basis, in producing dishes of cells with transformed foci (94% at 1.0 micrograms/ml). Benz(e)aceanthrylene, and benz(l)aceanthrylene produced 58% and 85% of the dishes with foci respectively at 10 micrograms/ml.