Interactions between subfractions of ox thymus H1 histone and either linear T7 DNA or superhelical PM2 DNA were studied by measuring the circular dichroism of H1:DNA complexes. H1 subfractions differed from one to the next in their effectiveness at distorting the circular dichroic spectrum of DNA by as much as 3- to 4-fold for both forms of DNA. The order of effectiveness of the subfractions was the same at all ionic strengths between 0.05 M and 0.25 M, but the degree of spectral distortion caused by any of the subfractions was sensitive to the salt concentration. At 0 M NaCl and above 3 M NaCl, there was little or no distortion of the spectrum of DNA by any subfraction; the maximum effectiveness for all of the subfractions was at 0.15 M to 0.2 M NaCl whether the DNA was linear or superhelical. Between 0 M and 0.15 M NaCl, the H1 subfractions in free solution underwent a conformational change from a substantially unfolded state to one that is presumably the native state. This was revealed by circular dichroism. In part, this folding of the protein molecules must account for the effect of salt on the ability of H1 to distort the circular dichroism of DNA when the two macromolecules are brought together in complex formation. The distortion of the circular dichroism of DNA by H1:DNA complex formation is thought to be due to side-by-side aggregation of fibers in an asymmetrically ordered array. Apparently, the different H1 subfractions induce formation of H1:DNA complexes that differ in degree of orderliness or in a more complicated geometric parameter of the array, and this is true for superhelical as well as linear DNA.