Abstract We have examined heat-stable opsonins to 4 species of gamma-irradiated mycobacteria ( M. tuberculosis (H37Rv), M. avium (28A), M. scrofulaceum and M. leprae (cd 103)) in complement-depleted sera collected from Indonesian subjects with tuberculosis (106 patients), leprosy (24 patients) and controls (40 hospital workers and 41 factory workers) indirectly by microtitre plate chemiluminescence (CL) assay and compared the results with antibody levels. The results indicate that there is a wide range of opsonic capacity for mycobacteria in complement-depleted sera. There was a poor correlation between the opsonic capacity as measured by CL and the anti-mycobacterial antibody content of sera measured by ELISA, suggesting that anti-mycobacterial antibody has little influence on the uptake of mycobacteria. However, a non-specific heat-stable opsonin appears to be present in some sera. Conversely, some sera from tuberculosis or leprosy patients suppress the production of reactive oxygen species from normal phagocytes in vitro when stimulated with M. tuberculosis. The relevance of this inhibition and the presence of heat-stable opsonins to the pathogenesis of tuberculosis have yet to be determined, but it is possible that the presence of opsonins may inhibit dissemination of tubercle bacilli to other organs.