An extracellular enzyme which utilizes molecular oxygen to oxidize cellodextrins to the corresponding aldonic acids has been isolated from culture filtrates of the white-rot fungus Sporotrichum pulverulentum. This enzyme, tentatively named cellobiose oxidase, has been highly purified by classical techniques and has been demonstrated to be a glycoprotein with a molecular weight of approximately 93000. Ultraviolet spectra of the enzyme in the presence and absence of substrate are characteristic of a hemoprotein. Acidic hydrolyses of the enzyme followed by a spectrofluorimetric investigation of the hydrolysate has demonstrated the presence of approximately one flavin component per enzyme molecule. The possible role of this complex enzyme in cellulose degradation is discussed.