A specific oxidoreductase converting aclacinomycin A to a new analog, aclacinomycin Y, was purified to apparent homogeneity from the culture filtrate of aclacinomycin-producing microorganisms. The isolated enzyme was a weakly acidic protein (isoelectric point, 5.9) with a molecular weight of about 72,000. The enzymatic reaction requires molecular oxygen and has a pH optimum at 5.5. The enzyme catalyzed an oxidation of the terminal sugar, L-cinerulose, of the trisaccharide moiety of aclacinomycin A to L-aculose (2,3,6-trideoxyhex-2-enopyranos-4-ulose) with removal of two electrons. Studies of substrate specificity revealed that the enzyme is an oxidoreductase capable of modifying anthracyclic triglycosides by oxidizing their terminal sugars.