[en] Highly purified suspensions of Mycobacterium leprae show a progressive increase in incorporation of [³H]thymidine and [³H]DOPA in short-term cultures as shown by scintillation counting. The intact bacilli are known to have a high permeability barrier. The experiments described suggest that [³H]DOPA becomes trapped within this barrier and oxidized inside the bacilli. Tests by pre-treatment with diethyl dithiocarbamate (DDC inhibitor of DOPA), cold DOPA or hyaluronidase distinguish the uptake of [³H]DOPA by bacilli from the effects of connective tissue contamination. Similar increases in labelling of bacilli by scintillation counting of cultures, have been observed by autoradiography of the organisms. The scintillation method shows promise for rapidly identifying drug resistance in lepromatous patients relapsing while on treatment with dapsone (DDS) rifampicin, clofazimine or other anti-leprosy drugs. (author)