Reaction of Mucor miehei protease with tetranitromethane was accompanied by the modification of about 7 tyrosine residues and the formation of a polymeric product. Studies of the nitrotyrosine content and enzymatic activity of the monomeric fraction suggested that 2 residues could be modified without a significant loss in proteolytic activity. Further reaction resulted in decrease in specific activity and changes in conformation. Based on the spectrophotometric titration data 2 groups of tyrosine residues were readily discernible; 8 which titrated freely with a pK app of 10.10 and 12–13 which were “buried” with a pK app of 11.36.