The mutagenic activities of the enantiomers of the diastereomeric pair of bay-region 3,4-diol-1,2-epoxides of the nitrogen heterocycle, dibenz[c,h]acridine, have been evaluated in histidine-dependent strains of Salmonella typhimurium and in an 8-azaguanine-sensitive line of Chinese hamster cells. In strains TA 98 and TA 100 of S. typhimurium the pair of enantiomers with [1R,2S,3S,4R] and [1S,2R,3R,4S] absolute configuration and the benzylic hydroxyl group trans to the epoxide oxygen are 2 to 4 times more mutagenic than the [1S,2R,3R,4R] and [1R,2S,3R,4S] isomers in which the benzylic hydroxyl and epoxide oxygen are cis. In both strains of bacteria there is very little difference in mutagenic activity between the enantiomers of each diastereomer. In contrast to these results in bacteria, the bay-region 3,4-diol-1,2-epoxide isomer with [1R,2S,3S,4R] absolute configuration is 5 to 7 times more mutagenic to Chinese hamster V79 cells than are the other 3 isomers. The enantiomeric pair of bay-region tetrahydro-1,2-epoxides of dibenz[c,h]acridine are at least 7 times more mutagenic than the diol-epoxides in the Salmonella assay, and no difference in mutagenic activity is observed between enantiomers. In the Chinese hamster V79 cell system, however, the tetrahydro-1,2-epoxide with [1R,2S] absolute configuration is 2- to 3-fold more mutagenic than its enantiomer with [1S,2R] absolute configuration. Homogeneous rat liver epoxide hydrolase does not catalyze the hydration of the diol-epoxide isomers to nonmutagenic products, although the tetrahydroepoxides, especially the tetrahydro-3,4-epoxide, are metabolized by the enzyme. Results of metabolic activation experiments with the bacterial mutagenesis system and microsomes from Aroclor 1254-treated rats are consistent with the mutagenicity data described above and support the concept that dibenz[c,h]acridine is metabolically activated to a bay-region diol-epoxide. Notably: (a) 3,4-dihydrodibenz[c,h]acridine, the expected precursor of a bay-region tetrahydroepoxide, is metabolized to a potent mutagen; (b) racemic dibenz[c,h]acridine 3,4-dihydrodiol is metabolized to products which are several-fold more mutagenic than are products of the metabolism of dibenz[c,h]acridine or its 1,2- or 5,6-dihydrodiols; and (c) the tetrahydro-3,4-diol, which lacks the isolated bay-region double bond, is not metabolically activated to a bacterial mutagen.