A study of the complexes formed between short linear DNA and three H1 variants, a typical somatic H1, and the extreme variants H5, from chicken erythrocytes, and spH1 from sea urchin sperm, has revealed differences between H1, H5 and spH1 that have implications for chromatin structure and folding. 1. All three histones bind cooperatively to DNA in 35 mM NaCl forming similar, but not identical, rod-like complexes. With sufficiently long DNA the complexes may be circular, circles forming more easily with H5 and spH1 than with H1. 2. The binding of H5 and spH1 to DNA is cooperative even in 5 mM NaCl, resulting in well-defined thin filaments that appear to contain two DNA molecules bridged by histone molecules. In contrast, H1 binds distributively over all the DNA molecules in 5 mM NaCl, but forms short stretches similar in appearance to the thin filaments formed with H5 and spH1. Rods appear to arise from the intertwining of regular thin filaments containing cooperatively bound histone molecules on raising the NaCl concentration to 35 mM. 3. The compositions of the rods correspond to one histone molecule for about every 47 bp (H1), 81 bp (H5) and 112 bp (spH1), suggesting average spacings of 24 bp (H1), 41 bp (H5) and 56 bp (spH1) in the component thin (double) filaments. Strikingly, these values are proportional to the linker lengths of the chromatins in which the particular H1 variant is the main or sole H1.