Our previous studies demonstrated that Mycobacterium leprae contains a characteristic o-diphenoloxidase which converts a variety of phenolic compounds to quinones in vitro. This enzyme was not present in any other mycobacteria tested. The results reported here deal with the uptake and binding of radioactive DOPA by M. leprae. The leprosy bacilli incubated with tritium-labelled DOPA, readily took up the substrate. The binding of DOPA by the bacilli was markedly inhibited by diethyldithiocarbamate. The organisms also bound tritiated norepinephrine. Mycobacterium phlei which does not oxidize phenolic substrates failed to bind DOPA. Cultures of melanocytes which contain o-diphenoloxidase took up tritiated DOPA. Catecholamine metabolism is known to be important in myocardial cells. Cultures of turtle-heart cells did not oxidize DOPA to quinone; however, these cells bound the labelled substrate. A cell line of fibroblasts derived from armadillo skin neither oxidized nor took up DOPA. The results indicate that, like melanocytes and turtle-heart cells, M. leprae probably possesses specific receptor sites for the binding and subsequent metabolism of phenolic substrates.