Mycobacterium leprae extracted and purified from experimentally infected armadillo was coated with rabbit sera raised against the total antigens of the following species of mycobacteria: M. leprae, M. avium, M. bovis BCG, and M. fallax. In addition, the bacteria were also coated either with serum from a lepromatous (LL), or a tuberculoid (TT) leprosy patient. The effectiveness of surface coating was verified by electron microscopy, with the aid of gold immunolabelling. The coated bacilli were phagocytized by mice bone marrow-derived macrophages, and the phagosome-lysosome fusions (PLF) were assessed during phagocytosis using acid-phosphatase (AcPase) cytochemistry. As compared to control preparations (like-wise treated with non-immune serum), significant but partial reversion of PLF inhibition was observed in all cases except when bacteria had been incubated with M. fallax antiserum (rapidly growing, non-pathogenic species). The results obtained suggest that some of the antimycobacterial antibodies may offer partial protection to the host during early events of infection by reverting the usual pattern of inhibition of PLF in infected macrophages.