Publisher Summary This chapter discusses the determination of flavine adenine dinucleotide (FAD) that depends on the specific reactivation of the apoenzyme of D-amino acid oxidase from pig kidney by this coenzyme. Comparison of the reactivation with a standard solution of FAD is necessary because the Michaelis constant of the enzyme for F AD varies with different preparations and temperatures. For pure FAD preparations, the ratio of the optical densities at 260 and 450 mμ (pH 7) is exactly 3.25. Solutions containing free FAD can be analyzed directly. Because biological material must be diluted considerably to obtain a final FAD concentration of 10−7 to 10−6 M, interference from other compounds present in the sample can usually be ignored.