Treatment of a native protein inhibitor of proteinases by tetranitromethane results inmodification of 3 (out of 8) tyrosine residues in each of the two subunits within the inhibitor molecule. Nitration of surface tyrosines does not change the corformation of the protein and has no effect on its ability to inhibit chymotrypsin. At the same time the tetranitromethane-treated inhibitor possesses a decreased activity with respect to trypsin. In the presence of 0,5 M DS-Na practically all tyrosine residues of the protein are nitrated.