The reactivity of the amino groups of histones in chromatin towards acetic anhydride was determined as a function of pH. In the pH range 7-10 the vast majority of amino groups in all five histones are buried. However, at higher pH values some of the histone amino groups become exposed, and the higher the lysine:arginine ratio for the histone the greater was the degree of unmasking observed. At pH 11.8 histone I appears to be completely dissociated, histones IIB1 and IIb2 have approx. 55% of the amino groups unmasked, and histones III and IV have approx. 25% of the amino groups unmasked.
