Chemical modification of chymotrypsin has led to the identification of several amino acid side-chains which are probably constituents of the active site of the enzyme. A single seryl and a single histidyl residue appear to cooperate in catalyzing the bond-breaking process while one or more tryptophanyl residues may be involved in the specific binding of substrate. Neither of the two methionyl residues is essential for enzyme activity although changes in kinetic properties occur when they are modified by oxidation or alkylation.