A study of the interaction with calf thymus DNA is described of a novel set of chiral ternary complex cations of general form [Ru(N4-tet) (phdi)]2+ (where N4-tet is the chiral linear tetradentate R(*)R(*)-picchxn or R(*)R(*)-picchxnMe2). Individual equilibrium binding constants (K(B)) have been determined from spectroscopic titrations employing the hypochromism induced in the visible absorbance of the cations on interaction with the nucleic acid. These demonstrate both stereo- and enantioselectivity in the binding interactions. These K(B) data, together with induced circular dichroism and DNA thermal denaturation results, are all indicative of selective intercalation of the bidentate components of the cations into the nucleobase stack of the duplex. Supportive evidence for a secondary binding mode for the picchxn complexes is provided by the different mutagenicity profiles obtained for related cations.