The antigen complex of A60 of Mycobacterium bovis BCG was analyzed by different immunological techniques to assess its relevance to tuberculosis and the involvement of its components in the immune reactions elicited in humans by tuberculous infection. A60 is composed of about 30 components, of which 8 were identified by available monoclonal antibodies (lipoarabinomannan, a glycolipid, and proteins of 65, 40, 38, 35, 19, and 14 kDa). The majority (87.5%) of anti-mycobacterial antibodies in sera from tuberculosis patients was directed against A60. Western blot (immunoblot) analysis indicated that the majority of the highly antigenic proteins present in mycobacterial homogenates were components of the A60 complex. A small percentage (7.8%) of A60 epitopes proved to be species specific. Thus, A60 proteins of 66, 41, 38, 37, 35, 34, 32, and 22 kDa were found to contain B-cell epitopes specific for M. bovis and not shared by Mycobacterium leprae oR Mycobacterium avium.