The red, cationic complex 2-hydroxyethanethiolato(2,2',2"-terpyridine)platinum(II), [(terpy)-Pt(SCH2CH20H)J+, binds strongly to DNA by a mechanism involving intercalation. By means of fluorescence spectroscopy, the platinum complex was shown to inhibit competitively the binding of the intercalating dye ethidium bromide to calf thymus DNA. This platinum complex increases the viscosity of calf thymus DNA, raises the melting temperature by up to 5°, and exhibits induced circular dichroism when bound to the DNA. The closed circular viral DNA from bacteriophage PM2 is unwound by the complex in a manner that is similar to that of ethidium bromide and with an unwinding angle that appears to be slightly less than that of ethidium. Studies of the related complex [(terpy)PtCll+, which has a substitutionally more labile chloride ligand, suggest that it also intercalates, especially at [DNA-PJ: [PtI ratios greater than 2. The potential utility of these new metallointercalation reagents as heavy atom probes in fiber diffraction or electron microscopic studies of the interaction process is discussed.