A flavoprotein[long dash]the coenzyme-factor (diaphorase) descr. by Straub, Corran and Green in Nature [London] vol. 143, 119, 1939[long dash]showing strong green fluorescence in soln. and having absorption maxima at 274, 359 and 451 m[mu], was isolated by following the protein fraction to which the flavinadenine dinucleotide (coenzyme of the d-aminoacidoxidase) is bound. Washed pig''s heart muscle tissue is ground with sand and Na2HPO4 soln., then centrifuged. The supernatant is precipitated with acetate buffer (pH 4.6). The precipitate is resuspended in 2% (NH4)2SO4 (containing 3% ethyl alcohol) and heated to 43[degree] for 10 min., then centrifuged. From the supernatant the flavoprotein is adsorbed to alumina C and eluted with Na2HPO4 soln. After dialysis the soln. is heated to 60[degree] for 5 min. and then fractionated with (NH4)2SO4. The final product contains 0.54% lactoflavin. The prosthetic group is the flavinadenine dinucleotide. Yield: 50-60 mg. flavoprotein from 1 kg. (wet) washed muscle.