D-amino-acid oxidase (DAAO) is a peroxisomal enzyme which uses flavin adenine dinucleotide (FAD) as a cofactor and oxidizes D-amino acids to the corresponding imino acids, producing ammonia and hydrogen peroxide. Its substrates include a wide variety of D-amino acids, but it is inactive on the naturally occurring L-amino acids. It has been suggested that it is involved in acid base balance in the kidney or it could act as a detoxifying agent which removes D-amino acids accumulated during aging. Recombinant human DAAO protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.