Susceptibility to infection with Mycobacterium leprae, the causative organism of leprosy, is the result of a defect in cell-mediated immunity (CMI). The co-operation of macrophages and T lymphocytes is known to be essential for competent CMI response. In this study we have examined peripheral blood monocytes from a range of leprosy patients in an attempt to identify a possible defect in macrophage function. The ability of these cells to produce hydrogen peroxide and superoxide, two bactericidal metabolites of the monocyte/macrophage, has been measured. Monocytes from leprosy patients were found to be capable of producing normal amounts of hydrogen peroxide and superoxide, and no differences in production were found between tuberculoid, lepromatous and control monocytes. These results suggest that macrophages in leprosy are competent, and that probably a T lymphocyte defect contributes to susceptibility to this disease.