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Conformational analysis of DNA–basic polypeptide complexes, based on stereo-chemical criteria and energy calculations and experimental results, is reported. Three types of polypeptide conformations were selected: the distorted β conformation similar to that proposed by Feughelmann et al.; a structure characterized by a repetition of a right-handed and left-handed α helix-type conformation; and an intermediate structure. A model of the complex between DNA and basic polypeptides was proposed, where the polypeptide chain fits the narrow groove of the B form of DNA. Van der Waals, hydrogen bond, electrostatic, and hydrophobic forces cooperate to stabilize the association complex. This structure also seems to be suitable to represent the molecular model of nucleoprotamines. In the case of nucleohistones, both grooves of DNA are involved in the interaction with the proteins. These have the nonbasic-rich portion in the α-helical conformation, whereas the part where a greater proportion of basic amino acids occurs presents a structure similar to nucleoprotamines and complexes between DNA and basic homopolypeptides. The distribution of basic residues of the F2A1(IV) histone on DNA is markedly disproportionate for the two opposite cylindrical sectors of the double helix. This suggests a new mechanism of supercoiling in nucleo-histones.