D-amino acid oxidase From Rhodotorula gracilis is a FAD-containing enzyme that belongs to the oxidase class that is characterized by the ability of the reduced Hal in to react quickly with oxygen, yielding hydrogen peroxide and the oxidized cofactor. Hydrogen peroxide, necessary for the production of glutaryl-7-ACA from cephalosporin C had a deleterious effect on the enzyme. H2O2 induced the oxidation of tryptophan and cysteine residues of the protein that could be involved in the dimerization process, required for the attainment of a fully competent enzyme. H2O2 had also a kinetic effect on the reaction catalyzed by D-amino acid oxidase. It was a pure noncompetitive inhibitor; the corresponding inhibition constants were K-is = 0.52 mM and K-ii = 0.70 mM.