Conformational alterations induced in DNA by the binding of various bivalent and tetravalent platinum complexes were characterized by means of differential pulse polarography and circular dichroism spectroscopy. It was found that at low levels of binding the platination of DNA markedly influenced its polarographic behaviour. The results indicated that the binding of the active anti-tumour complexes resulted in minor conformational changes in DNA when the double-stranded structure remained conserved. On the other hand, the attack by inactive anti-tumour compounds induced more severe alterations which had the character of denaturation of longer regions of the DNA molecule. It was also demonstrated that active anti-tumour tetravalent platinum complexes could react with DNA, without their prior reduction to the bivalent state, and may induce in DNA conformational changes similar to those produced by bivalent cisplatin.