Many polycyclic aromatic hydrocarbons containing peripherally fused cyclopenta rings are believed to be activated primarily by epoxidation of the cyclopenta ring. The cyclopenta epoxides of a series of four cyclopenta benzanthracene derivatives, benz[e]aceanthrylene-5,6-oxide, benz[j]aceanthrylene-1,2-oxide, benz[l]aceanthrylene-1,2-oxide and benz[k]acephenaceanthrylene-4,5-oxide were synthesized from their parent hydrocarbons by formation of the bromohydrin followed by dehydrobromination, and characterized by u.v.-vis, and 1H n.m.r. spectroscopy and mass spectrometry. The mutagenicity of these compounds was investigated in the Ames plate incorporation assay with Salmonella typhimurium strain TA98. All the oxides were active without exogenous metabolic activation (170-320 His+ revertants per nanomole) and also toxic above 0.5 microgram/plate. Addition of S9 protein did not increase, and generally decreased, the mutagenicity of the oxides, while toxicity was largely unchanged. These results are consistent with the postulated role of cyclopenta oxides as major contributors to the mutagenicity of the parent compounds in the Ames assay.