The granulomatous skin lesions in leprosy are thought to be initiated by the immune response to certain antigens of the causative agent, Mycobacterium leprae. The antigen 85 complex is one of the major targets in the immune response to M. leprae infection. In the present study, a panel of previously characterized monoclonal antibodies (MAbs) (3A8, Rb2, A4g4, A2h11, Pe12, and A3c12) reacting with different epitopes of the 85 complex proteins of Mycobacterium tuberculosis and M. leprae was employed in a comparative immunohistological analysis to demonstrate the in situ expression of 85 complex antigenic epitopes in leprosy lesions across the clinical spectrum and in M. leprae-infected armadillo liver tissues. These MAbs showed a heterogeneous staining pattern in a given leprosy lesion. In highly bacilliferous borderline and lepromatous leprosy lesions, MAbs Rb2, A4g4, A2h11, and Pe12 stained clear rod-shaped M. leprae bacilli within macrophages, and the degree of staining correlated with the bacillary index of the lesion. On the other hand, MAbs 3A8 and A3c12 staining was mostly seen as a diffuse staining pattern within interstitial spaces and on the membranes of the infiltrated cells but not the bacilli. In paucibacillary borderline and tuberculoid leprosy lesions, only 3A8, Rb2, and A3c12 showed distinct staining in association with infiltrates in the granuloma. None of these MAbs showed any detectable reaction with control nonleprosy skin lesions, while MAb A3c12 positively stained the granulomas of both leprosy and control specimens. In situ reactivity of these MAbs with M. leprae-infected armadillo liver tissues also showed a heterogeneous staining pattern. Interestingly, a clear difference in expression of these epitopes was observed between armadillo tissues and human leprosy lesions. By immunogold ultracytochemistry, we further showed the differential localization of these MAb-reactive epitopes on the cell surface, in the cytosol, and at the vicinity of M. leprae within Kupffer cells of armadillo liver tissues. Our results indicate that these antigenic epitopes of the antigen 85 complex are differentially expressed in leprosy lesions and infected armadillo tissues and that they could be target determinants in the immunopathological responses during M. leprae infection.