The interaction of the platinum(II) polypyridine complexes [Pt(bipy)(2)](2+), [Pt(quaterpy)](2+), [Pt(terpy)(n-Rpy)](2+) and [Pt(bipy)(py)(2)](2+) (bipy = 2,2'-bipyridine; terpy = 2,2':6',2 "-terpyridine; quaterpy = 2,2':6',2 ":6 ",2'''-quaterpyridine; n-R = H, 2-CH3, or 4-CH3) with double-helix DNA has been studied with a variety of experimental techniques. Induced circular dichroism, strong hypochromism and red shifts of the absorption maxima of the complexes, increase in melting temperature and viscosity of DNA, and inhibition of the reaction of the complexes with thiourea in the presence of DNA, characterize the processes. Intercalation, implying the whole molecule or part of it, is the suggested binding mode. The binding constants, K-B, determined spectrophotornetrically at 25 degrees C, pH 7, and I = 0.15 M, using the McGhee-von Hippel approach, increase in the order [Pt(bipy)(py)(2)](2+) < [Pt(terpy)(py)](2+) < [Pt(quaterpy)](2+), on increasing aromatic planar surface extension. The steric interference with double helix of the methyl group in [Pt(terpy)(2-Mepy)](2+) destabilizes the interaction by reducing the stacking surface.