Mycobacterium leprae cells extracted from the skin biopsies of 14 bacilliferous lepromatous patients were maintained in human-murine macrophage cultures for 3 weeks in the presence of [3H]thymidine and DDS (4,4'-diaminodiphenyl sulfone). All cultures except one containing freshly extracted viable bacilli showed significant incorporation of [3H]thymidine as compared to control cultures containing heat-killed bacilli of the corresponding strain. Six susceptible strains of M. leprae obtained from untreated, freshly diagnosed patients showed significant inhibition of the uptake of the radiolabel in the presence of 3 and 10 ng of DDS per ml per culture. Eight strains of M. leprae obtained from patients clinically suspected of DDS resistance were tested in a similar manner. These strains were also concurrently inoculated in the footpads of mice given orally 10(-2), 10(-3), and 10(-4) g of DDS per 100 g of body weight for 9 months. Concordant results were obtained by both methods: five strains were found to be resistant, one was susceptible, and one was partially resistant. Strain VIII did not incorporate [3H]thymidine in the macrophage cultures and proved to be resistant in the mouse footpad. The macrophage culture system provides a sensitive, rapid screening method for the early diagnosis of DDS resistance.