The core protein produced by mild proteolytic digestion of the lac repressor has been purified on phosphocellulose, The repressor and core proteins were reacted with the sulfhydryl specific reagents, 2-chloromercuri-4-nitrophenol and fluorescein mercuric acetate. Modification of the cysteine residues did not alter the affinity of the proteins for inducer molecules. The operator binding activity of both proteins was unaffected by the reaction with 2-chloromercuri-4-nitrophenol; however, this binding was essentially abolished upon modification with fluorescein mercuric acetate. This loss of operator DNA binding activity in response to modification supports the thesis that determinants for specific DNA binding are located in the core region of the protein. Fluorescence spectral studies on repressor modified with 2-chloromercuri-4-nitrophenol and fluorescein mercuric acetate were performed. The quenching observed upon titration of repressor with either reagent indicates that energy transfer was occurring between the protein tryptophans and the cysteine-conjugated chromophores. Inclusion of dithiothreitol during the titration prevented the labelling of the cysteines; a corresponding decrease in energy transfer was seen. The addition of of inducer produces a blue shift in the repressor emission spectrum, but did not affect the quenching process. The quenching was sensitive to dithiothreitol for the repressor-inducer system as it had been for the repressor protein alone. The spectral behavior of the core protein was essentially identical to that displayed by the repressor.