The influence of dimethylsulfoxide on the interaction of the DNA molecule with divalent platinum coordination compounds in solution was studied by the circular dichroism, dynamic birefringence, and low-gradient viscometry methods. It was shown that the presence of dimethylsulfoxide in solution could influence the spectral properties of the DNA molecule, but its low concentration had no effect on the character of binding of DNA with cis- and trans-dichlorodiamminoplatinum. The introduction of dimethylsulfoxide in the first coordination sphere of platinum did not prevent binding of such compounds to complexes with DNA. The [Pt(en)Cl(CH₃)SO]Cl compound was shown to form a coordination bond with DNA at position N7 of guanine and compete for the binding site with cis-dichlorodiamminoplatinum in complex coordination compounds. Complex formation hindered the protonation of double helical DNA.