The interaction of DNA with cis-diaminodichloroplatinum at different ionic strengths was studied by dynamic birefringence, viscosimetry, and circular dichroism. Although cis-diaminodichloroplatinum is a nonelectrolyte, it can bind to DNA by electrostatic interaction. It was found that the mechanism of complex formation is determined by the [Pt]/[DNA] ratio. At a low concentration, cis-diaminodichloroplatinum formed complexes with the phosphate groups of DNA, while at a higher one it bound to the nucleic bases. As this took place, the secondary structure of DNA did not change. An increase of cis-diaminodichloroplatinum concentration disrupted the stacking contacts and denatured the DNA. It was assumed that the stability of the complex is ensured by incorporation of the nitrogen-containing groups of the bases into the platinum first coordination sphere. The phosphates of DNA served as counterions in the outer coordination