The interaction of Rh2(II)(acetate)4, cis-[Rh(III)(en)2Cl2] Cl (en = ethylenediamine) and [Rh(III) (NH3)5Cl]Cl2 with calf thymus DNA has been studied at various r values [formula; see text] and interaction times. Electronic spectra, melting and cooling curves and sedimentation data indicate no interaction of the acetate complex with DNA, except in the case of a high r value and long interaction time. The other two complexes have been found to interact with the phosphate groups, thus stabilizing the macromolecule.