Polyclonal antibodies that bind selectively to DNA modified by antitumour cisplatin and its analogues were isolated. The reactivity of the antibodies with the epitope was enhanced by thermal denaturation of DNA that had been modified by cisplatin before its denaturation. On the other hand, denaturation of DNA before its modification resulted in considerably less reaction of the antibodies. The conversion of monofunctional cisplatin-DNA adducts to bifunctional lesions increased the capability of the modified DNA to competitively inhibit the antibodies. The double-helical oligonucleotides containing a unique bifunctional adduct formed by cisplatin at the d(GG) site cross-reacted with the antibodies in contrast to the oligonucleotide containing a single monofunctional adduct formed at the d(G) site. In addition, poly(dG-dC) . poly(dG-dC) modified by cisplatin did not react with the antibodies. It was concluded that the antibodies recognized monodentate lesions, intrastrand cross-links between two purine nucleosides separated by one or more nucleosides and interstrand cross-links negligibly. The antibodies apparently recognized a chemical nature of the bifunctional adduct formed between two adjacent purines and not an unusual conformational feature of DNA resulting from the formation of this adduct. The antibodies were used to analyse the adducts formed by cisplatin on DNA of cultured cells exposed to this drug. During the subsequent incubation of the already exposed cells in the drug-free medium, a part of the bifunctional adducts of cisplatin was completely removed from DNA or transformed to the adducts not recognized by the antibodies.