The nature of the complexes formed between histone H1 and linear double-stranded DNA is dependent on ionic strength and on the H1 : DNA ratio. At an input ratio of less than about 60% (w/w) H1 : DNA, there is a sharp transition from non-co-operative to co-operative binding at a critical salt concentration that depends on the DNA size and is in the range 20 to 50 mM-NaCl. Above this critical ionic strength the H1 binds to only some of the DNA molecules leaving the rest free, as shown by sedimentation analysis. The ionic strength range over which this change in behaviour occurs is also that over which chromatin folding is induced. Above the salt concentration required for co-operative binding of H1 to DNA, but not below it, H1 molecules are in close proximity as shown by the formation of H1 polymers upon chemical cross-linking. The change in binding mode is not driven by the folding of the globular domain of H1, since this is already folded at low salt in the presence of DNA, as indicated by its resistance to tryptic digestion. The H1-DNA complexes at low salt, where H1 is bound distributively to all DNA molecules, contain thickened regions about 6 nm across interspersed with free DNA, as shown by electron microscopy. The complexes formed at higher salt through co-operative interactions are rods of relatively uniform width (11 to 15 nm) whose length is about 1.6 times shorter than that of the input DNA, or are circular if the DNA is long enough. They contain approximately 70% (w/w) H1 : DNA and several DNA molecules. These thick complexes can also be formed at low salt (15 mM-NaCl) when the H1 : DNA input ratio is sufficiently high (approximately 70%).