Mycobacterium leprae separated from infected human tissues, oxidizes 3,4-dihydroxyphenylalanine (dopa) and a few other phenolic compounds1–3. This property is not shown by several other mycobacteria1,2,4. Phenoloxidase occurs in vertebrate melanocytes and is widely distributed in the plant kingdom5. (E.C. 1.10.3.1, o-diphenol: oxygen oxidoreductase). For a large number of substrates the phenoloxidase of the leprosy bacilli is similar to the enzyme from plant sources, and distinct from mammalian phenolase, which has a limited substrate specificity6. When the reaction products were compared, another distinguishing feature of the M. leprae enzyme became evident and this is reported here.