Initial studies on the mutagenicity and metabolism of a novel cyclopenta-PAH, benz(j) aceanthrylene, are reported in the Salmonella bacterial system. The spectrum of activity of benz(j)aceanthrylene over the 5 Ames tester strains is similar to that of benzo(a)pyrene, and the dose-response curves for strain TA98 are comparable. Like other biologically active PAH, benz(j)aceanthrylene is a frame-shift mutagen requiring metabolic activation. An interesting feature of the S9 dependence of activity is the low concentration (about 10-fold smaller than for benzo(a)pyrene) at which optimal activity is observed. The 1,2-dihydro-1,2-diol (product of metabolism of the cyclopenta-ring) appears to be the predominant metabolite, and implicates the 1,2-oxide as the ultimate mutagenic species.