A factor has been identified in extracts from human HeLa and hamster V79 cells that retards the electrophoretic mobility of several DNA restriction fragments modified with the antitumor drug cis-diamminedichloroplatinum(II) (cisplatin). Binding of the factor to cisplatin-modified DNA was sensitive to pretreatment with proteinase K, establishing that the factor is a protein. Gel mobility shifts were observed with probes containing as few as seven Pt atoms per kilobase of duplex DNA. By competition experiments the dissociation constant, Kd, of the protein from cisplatin-modified DNA was estimated to be (1-20) X 10(-10) M. Protein binding is selective for DNA modified with cisplatin, [Pt(en)Cl2] (en, ethylenediamine), and [Pt(dach)Cl2] (dach, 1,2-diaminocyclohexane) but not with chemotherapeutically inactive trans-diamminedichloroplatinum(II) or monofunctionally coordinating [Pt(dien)Cl]Cl (dien, diethylenetriamine) complexes. The protein also does not bind to DNA containing UV-induced photoproducts. The protein binds specifically to 1,2-intrastrand d(GpG) and d(ApG) cross-links formed by cisplatin, as determined by gel mobility shifts with synthetic 110-bp duplex oligonucleotides; these modified oligomers contained five equally spaced adducts of either cis-[Pt(NH3)2d(GpG) or cis-[Pt(NH3)2d(ApG)]. Oligonucleotides containing the specific adducts cis-[Pt(NH3)2d(GpTpG)], trans-[Pt(NH3)2d(GpTpG)], or cis-[Pt(NH3)2(N3-cytosine)d(G)] were not recognized by the protein. The apparent molecular weight of the protein is 91,000, as determined by sucrose gradient centrifugation of a preparation partially purified by ammonium sulfate fractionation. Binding of the protein to platinum-modified DNA does not require cofactors but is sensitive to treatment with 5 mM MnCl2, CdCl2, CoCl2, or ZnCl2 and with 1 mM HgCl2.(ABSTRACT TRUNCATED AT 250 WORDS)