The interaction of copper(II/I) complexes of a few 2,9-dimethyl-1,10-phenanthrolines with calf thymus DNA has been investigated using absorption and circular dichroic spectral and electrochemical techniques and viscometry. The observation of the usual hypochromism and the novel hyperchromism in the absorption spectra of [Cu-I(bcp)(2)](+) [bcp = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline] and [Cu-I(dpsmp)(2)](3-)[dpsmp(2-)=2,9-dimethyl-4,7-bis(sulfonatophenyl)-1,10-phenanthroline] respectively in the presence of DNA and the increase in viscosity of DNA at low loadings of both these complexes have been interpreted in terms of bridging of a pair of DNA duplexes by the complex species. These tetrahedral copper(I) complexes, which lack minor groove binding because of substituents at the 4- and 7-positions of phen ring, are efficient in bridging the duplexes. The electrochemical behaviors of [Cu-I(dmp)(2)](+) [dmp = 2,9-dimethyl-1,10-phenanthroline] and [Cu-I(bcp)(2)](+) bound to DNA have been compared with that of the analogous sulfonated complex [CU(dpsmp)(2)](2-/3-). The DNA binding constants determined reveal that dpsmp(2-) complex is engaged in DNA binding less intimately than the bcp complex. While Coulombic interactions are clearly more important than other types of interactions for the former, nonclassical hydrophobic interactions for the latter. The Hill analysis of the absorbance data obtained as a function of added DNA reveals Hill coefficients greater than unity, which may be construed as evidence for cooperative binding of the copper complexes to B-DNA.