The interactions in buffered 0.14 M NaCl (pH 7.0) of DNA, native or sonicated, with histone H1 and with its fragments N-H1 (residues 1-72) and C-H1 (residues 73-COOH), have been studied by using the techniques of sedimentation, thermal denaturation and solubility. Histone H1 shows a preferential affinity for high molecular weight DNA in comparison with sonicated DNA. The binding of histone H1 to sonicated DNA in 0.14 M NaCl is reversible and the reversibility decreases with time. These findings are consistent with the view that H1 is cooperatively distributed along DNA molecules and that this distribution is energetically more favoured for long DNA than for sonicated DNA. It has been found that under conditions of moderate salt concentration, the C-terminal region is the main one responsible for the interaction of H1 with DNA, and also for a cooperative distribution of the histone along the DNA molecules. Addition of urea to the solution produces a decrease of solubility of H1-DNA complexes. This effect reflects an additional condensation of the complex, which is probably related to the unfolding of the globular part of the histone. It is suggested that this globular region does not play a substantial role in the condensation of H1-DNA complexes. A model which takes into account the presence of free lysyl and phosphate groups within the complex is discussed. © 1978 Masson, Paris.