Electrophoretic and sedimentation velocity studies on the histone H3–H4 complex show that provided the H3 cysteine residues remain reduced the complex reforms quantitatively when removed from a variety of denaturing conditions. If histone H3 is allowed to become intramolecularly oxidized while denatured only monomer and large aggregates are formed on return to native conditions. At pH 7 ionic strength 0.1 the complex remains with reduced sulfhydryl groups indefinitely suggesting a vital role for the sequence 96–110 in histone H3 in the tertiary structure of the complex.