The binding of sodium n-dodecyl sulphate (SDS) to calf thymus histone H2B was studied in the pH range 3.2-10 by equilibrium dialysis at 27 and 37 degrees C. The binding data have been used in terms of the Scatchard equation showing unusual plots with minima. No theoretical model gives Scatchard plots with such conditions, except for a combination of two types of binding with large differences in the Hill coefficients and binding affinity, i.e. a combination of negatively and positively cooperative binding sites.