The interaction of DNA with coordination compounds of divalent platinum and tri valent cobalt was studied. Complexes containing simultaneously different coordination compounds were prepared by various procedures. It was shown that the binding of trans-diaminodichloroplatinum (DDP) to DNA prevents its further complexing with cis-DDP, while in the presence of cis-DDP the possibility for trans-DDP to bind to DNA in solution remains. The study of similar DNA complexes with trans-DDP and cobalt hexamine indicated the competition for the binding sites of these compounds to the DNA molecule.