It is shown that the formation of complexes with several arginine peptides stabilizes the B-form of DNA with 10 (+/- 0.15) base-pairs per turn at all relative humidities, even upon complete dehydration. From an analysis of the packing arrangement and from the calculated diffraction patterns, it is concluded that arginine is associated with DNA in its major groove. It is also shown that the diffraction pattern of nucleoprotamine can be interpreted by placing the protamine on the major groove of DNA. The strong intensity on the first layer-line is due to the influence of neutral residues on the diffraction pattern. Thus, we conclude that protamine is bound to the major groove of DNA.