Abstract Complexes of natiye calf thymus DNA with the cationic polypeptides poly-lornithine, poly-l-lysine, poly-l-arginine and poly-l-homoarginine, have been prepared and their solubility, stoichiometry, absorption spectra and thermal denaturation studied. Increasing the peptide cation/DNA phosphate ratio, up to electrostatic equivalence, yielded progressively more insoluble products and increased the turbidity of the “soluble” fraction. Certain spectral changes were observed which may be largely attributed to anomalous scattering in the absorbing region. Addition of the polypeptides to DNA resulted in a marked stabilization of the helix against thermal denaturation. At peptide cation/DNA phosphate ratios less than electrostatic equivalence, thermal denaturation monitored at 260 and 280 mμ revealed a biphasic transition profile: the first transition had a melting temperature similar to DNA under the same solvent conditions; the second melting temperature was characteristic for the type of polypeptide in the complex. Thermal denaturation monitored at 350 mμ (i.e., turbidity transitions) showed a monophasic profile at the higher melting temperature of the DNA-polypeptide complex. The different polypeptides stabilized DNA against melting to different extents. In order of decreasing degree of stabilization they are: poly-l-ornithine > poly-l-lysine > poly-l-arginine > poly-l-homoarginine. Analysis of the dispersion of hyperchromicity demonstrated that poly-l-ornithine and poly-l-lysine preferentially stabilize A-T rich regions, whereas poly-l-arginine and poly-l-homoarginine appear less discriminating. The soluble DNA-polypeptide complexes could be subfractionated by ultracentrifugation into a fraction which melted like “naked” DNA, and a fraction which melted at the higher temperature characteristic of the complex and showed a peptide cation/DNA phosphate ratio close to electrostatic equivalence. The experimental data imply that, under proper conditions of annealing, the basic polypeptides form definable molecular structures with DNA; the binding reaction is stoichiometric and co-operative. Model-building suggests that the polypeptides could interact with either the large or small groove of DNA.