Complexes of DNA with cis-diamminedichloroplatinum and trans-diamminedichloroplatinum were studied by low-gradient viscometry, spectrophotometry, luminescence, dynamic light scattering, and fluorescence and atomic force microscopy. It was demonstrated that a DNA molecule decreases in size upon its interaction with platinum coordination compounds. The formation of DNA complexes with cis-diamminedichloroplatinum reduces the luminescence intensity and the constant of ethidium bromide binding with DNA, while in the case of trans-diamminedichloroplatinum, these effects turn out to be somewhat less pronounced.