Cyclopenta[cd]pyrene (CPP) is a ubiquitous environmental pollutant whose 3,4-epoxide (CPPE) is generally regarded as its ultimate carcinogenic metabolite. The present study was undertaken to determine the structures of major DNA adducts formed by CPPE in vitro. Incorporation of specific radiolabeled bases into calf thymus DNA prior to reaction with CPPE demonstrated that the major adducts were formed by guanine, while minor adducts were formed by adenine and cytosine. Unmodified DNA was reacted with [3H]CPPE and the deoxynucleoside adducts obtained were compared chromatographically with the products obtained by reaction of CPPE with 2'-deoxyguanosine (dGuo). Two dGuo adducts from the latter reaction were identified by 1H-NMR, fast atom bombardment mass spectrometry, and circular dichroism as diastereoisomers of cis-3-(deoxyguanosin-N2-yl)-4-hydroxy-3,4-dihydroCPP. Other products that may have included the isomeric trans-N2-dGuo adduct were formed in the reaction. The major adduct fraction in the DNA digest, accounting for over 70% of the total, was chromatographically indistinguishable from the two cis dGuo-N2 adducts. A second DNA adduct fraction was observed, which appeared also to be formed by reaction with guanine as indicated by experiments in which DNA containing [3H]guanine was reacted with unlabeled CPPE. The results confirm that guanine is the major target in DNA for reaction with CPPE and are the first proof of structure for a CPPE-deoxynucleoside adduct.