Interaction of DNA with cis- and trans-diaminodichloroplatinum (DDP), [(NH4)(4)Pt]Cl-2, and K2PtCl4 was studied. It was found that binding of cis- and trans-DDP and [PtCl4](2-) to DNA involves formation of aquacomplexes. The departing chlorine atoms play an important role in complex formation. For cis-DDP, two nucleic base groups are inserted into the platinum first coordination sphere. The complex is stabilized by the DNA phosphates shielding the positive charges in the outer coordination sphere. For trans-DDP, the localization of the platinum acido ligands and the amino groups does not favor formation of a bidentate,complex with the nucleic bases of DNA. The presence of two DNA-binding sites in the [PtCl4](2-) aquacomplexes does not stabilize the latter. No coordination bonds can be formed between [(NH)(4)Pt]Cl-2 and DNA, because the envelope groups of the [(NH)(4)Pt](2+) ion cannot be substituted. This ion interacts with DNA, but the mechanism of this interaction is different from that for DNA and cis-DDP.