A reliable screening technique for assessing the sensitivity ofMycobacterium leprae to drugs has been developed. The method is based on the susceptibility or otherwise ofM. leprae- infected tissues from lepromatous leprosy patients to the action of diaminodiphenyl sulphone (dapsone) or rifampicin on the incorporation of [14C]-acetate into lipids. The extent of inhibition or lack of inhibition correlated very well with the drug sensitivity or resistance of the bacteria isolated from the patients to the above drugs. A similar trend was observed when the incorporation into individual fractions of neutral lipids was measured. There was no incorporation by heat-killed tissues. This method correlates well with the 3,4-dihydroxyphenylalanine uptake studies.