Reaction of the lactose repressor protein from Escherichia coli with high molar excesses (up to 800 fold) of tetranitromethane resulted in modification of tyrosine residues in the amino-terminal and core regions of the molecule. Tyrosines 7 and 17 exhibit significant reactivity at low levels (5-10 fold molar excess) of tetranitromethane. The loss of operator binding activity upon nitration at these low concentrations of reagent indicates involvement of these two tyrosines in the binding process. Inducer binding activity was maintained at approx. 90% of unreacted repressor for all excesses of reagent studied. Addition of inducer to the repressor prior to reaction resulted in decreased modification of tyrosines in the core region, but anti-inducers did not affect the reaction significantly. The effect of inducers on the pattern of reaction apparently reflects the conformational change which occurs upon binding of these ligands. Acetylation of the repressor protein with N-acetylimidazole modified lysines and tyrosines with complete loss of operator binding activity and retention of 75-80% of inducer binding activity.