Our earlier studies indicated that the enzyme o-diphenoloxidase was absent in Mycobacterium leprae separated from depromatous human tissues. At that time the bacilli were not available from any other source. The existence or absence of this enzyme in M. leprae recovered from infected armadillo tissues were reinvestigated. The intact cells which were metabolically active, failed to oxidize DOPA. Likewise, DOPA and its derivatives were not oxidized by the enzymatically active cell-free preparations from M. leprae. Upon incubation of DOPA for more than 2 h with whole cell suspensions or particulate fractions, there was no development of colour with an absorption maximum of 540 nm as has been reported for an intermediate of DOPA oxidation. However, DOPA and several phenolic compounds were very actively oxidized by mushroom tyrosinase. The results suggested that M. leprae is deficient in o-diphenoloxidase, and this enzyme is not an intrinsic characteristic of this mycobacterium.