DNA represents the primary target for platinum antitumor metal complexes and is the probable target for newly developed cytotoxic gold(III) complexes. To test this hypothesis the reactions with calf thymus DNA of five representative gold(III) complexes-namely [Au(en)(2)]Cl-3, [Au(dien)Cl]Cl-2, [Au(cyclam)](ClO4)(2)Cl, [Au(terpy)Cl]Cl-2 and [Au(phen)Cl-2]Cl-were analyzed in vitro through various physicochemical techniques including circular dichroism, absorption spectroscopy, DNA melting, and ultradialysis, It is shown that all tested complexes interact with DNA and modify significantly its solution behavior. The solution conformation of DNA is affected to variable extents by the individual complexes as shown by CD titration experiments. Notably, in all cases, the gold(III) chromophore is not largely perturbed by addition of calf thymus DNA ruling out occurrence of gold(III) reduction. Ultradialysis experiments point out that the binding affinity of the various complexes for the DNA double helix is relatively low; in most cases the gold(III)/DNA interaction is electrostatic in nature and reversible. The implications of these findings for the mechanism of action of antitumor gold(III) complexes are discussed.