Active tuberculosis (TB) and leprosy are difficult to diagnose early because there are few organisms to detect and the specific immune response does not distinguish between active and inactive disease. We developed an immunoassay for lysozyme to see whether serum lysozyme levels could be used to identify individuals with clinical leprosy or TB. The immunoassay for lysozyme proved superior to standard enzyme assays that were less sensitive and reliable. The lysozyme assay was compared with assays for antibodies to Mycobacterium tuberculosis lipoarabinomannan (LAM) and M. leprae phenolic glycolipid-1. The sera tested were from Ethiopian leprosy (paucibacillary and multibacillary) and TB patients and from healthy Ethiopian and U.S. controls. The lysozyme assay was able to detect more of the individuals with TB (sensitivity, 100% for 19 patients) or leprosy (sensitivity, 86% for 36 patients) than either antibody assay. In particular, lysozyme levels were raised in a higher proportion of the paucibacillary leprosy patients (83% of 17), for whom the antibody assays were less sensitive; the LAM IgG and the phenolic glycolipid-1 IgM levels were raised in only 62 and 44% of 16 patients, respectively. The data suggest that lysozyme measurements may be useful in the diagnosis of mycobacterial infections and other chronic infectious granulomatoses.