Aceanthrylene (ACE), a cyclopenta-fused polycyclic aromatic hydrocarbon (CP-PAH) derivative of anthracene has been shown to be highly mutagenic in Salmonella typhimurium strain TA98 (1). C3H10T1/2CL8 (C3H10T1/2) mouse embryo fibroblasts have been used to study the metabolism and morphological transforming ability of PAH and several CP-PAH (2-5). The CP-PAH, cyclopenta(cd)pyrene and three of the four possible cyclopenta-benz(a)anthracene isomers were active in morphologically transforming C3H10T1/2 cells. Cyclopenta(cd)pyrene and two of the four possible isomers of cyclopenta-benz(a)anthracene, benz(j)aceanthrylene, and benz(l)aceanthrylene, were metabolized by C3H10T1/2 cells to dihydrodiol metabolites including cyclopenta-ring dihydrodiols. The study examines the ability of ACE, the smallest linear-fused CP-PAH yet examined, to be metabolized, to form DNA adducts, and to induce morphological transformation in C3H10T1/2 mouse embryo fibroblasts.