The DNA binding and interstrand cross-linking properties of the dinuclear platinum complex [{ cis-Pt(NH 3) 2Cl} 2bpsu](NO 3) 2 (bpsu is 4,4′-dipyridyl sulfide) (II) and the mononuclear complex [ cis-Pt(NH 3) 2Cl(4-methylpyridine)]NO 3 (I) were compared with those of [{ cis-Pt(NH 3) 2Cl} 2H 2N(CH 2) 4NH 2](NO 3) 2 (III) in order to understand the mode of action of complexes I and II. Both compound I and compound II caused significantly different changes of conformation in poly(dG-dC)·poly(dG-dC) than compound III did. Studies of DNA binding, interstrand cross-linking and fluorescence assay suggest that compound I monofunctionally binds to DNA and compound II bifunctionally binds to DNA, that the dinuclear platinum complex II more efficiently interacts with DNA compared to its monomeric analog, and that platinum I and II complexes both interact with DNA in a non-intercalative mode. All the results indicate that the mode of action of the dinuclear complex II is different from that of the mononuclear complex I.