Structural characterizations of the DNA adducts derived from reaction of the racemic bay region anti-diol epoxides of dibenz[a,j]anthracene and 7-methyldibenz[a,j]anthracene with calf thymus DNA are presented. Quantities of adducts necessary for spectroscopic characterization were obtained from reactions of the respective diol epoxides with individual deoxyribonucleotides. Both hydrocarbon diol epoxides showed similar adduct profiles upon reaction with calf thymus DNA in vitro which were composed mainly of three deoxyguanosine and four deoxyadenosine adducts. No significant modification of pyrimidine bases in DNA was detected with either of the diol epoxides. Approximately 3 times more deoxyguanosine than deoxyadenosine residues in the DNA were found to be modified by both diol epoxides. The DNA reactions showed very similar stereo- and enantioselectivities with both diol epoxides. The stereochemistries of addition of the purine bases to the diol epoxides were determined from analysis of the NMR spectra of individual adducts. The predominant adducts formed were products of trans addition of the exocyclic amino group of purines to the diol epoxides. The enantiomeric nature of the various adducts was determined from reaction of the individual deoxyribonucleotides with the pure (+)-anti-diol epoxide of dibenz[a,j]anthracene. The major deoxyguanosine and deoxyadenosine adducts from reactions with DNA were found to arise from the (+)-enantiomer of both hydrocarbon diol epoxides. The high reactivities of both diol epoxides (24-38%) with DNA in solution are consistent with the high tumor-initiating activity exhibited by the diol epoxide of dibenz[a,j]anthracene relative to the parent hydrocarbon.