Some quantum yields for the destruction of amino acids have been determined. The inactivation of the enzymes chymotrypsin, lysozyme, ribonuclease, and trypsin by ultraviolet light can be accounted for quantitatively by summing the products of (i) the probability that light is absorbed by a given amino acid residue, ∊ i, and (ii) the probability that absorbed light induces a chemical change, with a quantum efficiency φ i, in the residue. The principal residues involved are cystyl and tryptophanyl. Peptide bond rupture is not important. Analysis of inactivated enzymes verifies the assumption of the existence of several inactivation mechanisms.