Radiolabelled sonicate of Mycobacterium leprae when examined by SDS-PAGE and two-dimensional gel electrophoresis (2-DE) contained fewer antigens than the comparable sonicate from M. Bovis (bacillus Calmette-Guerin) (BCG). A solid-phase immunopurification assay with anti-M. leprae monoclonal antibodies (MoAb) was used to characterize four of these antigens. Three of the MoAb were M. leprae-specific and with them antigens with apparent molecular weights (Mr) of 12,000 (12K), 18K, and 35K were isolated. On 2-DE, the heavily labelled 12K antigen was heterogeneous with a range in pI of 4.8-5.2. The 35K antigen, which was identified by a conformational determinant, and the 18K antigen were also acidic proteins with pI of 5.4 and 5.1. The fourth antigen was purified from both M. leprae and BCG sonicates and had an Mr of 70K and a pI of 5.1. MoAb reacting with the cell wall protein of M. leprae resulted in separation of multiple bands ranging in Mr from 12K to 65K, rather than the dominant 65K protein seen in immunoblots. A similar pattern was obtained with MoAb that reacted with two cell wall polysaccharide antigens, and these antibodies may have co-precipitated the radiolabelled cell wall proteins. Immunoprecipitates of the M. leprae sonicate with human lepromatous leprosy sera, when analysed by 2-DE, were also found to contain the dominant 12K band and the 35K band. Furthermore, half the radiolabelled BCG antigens were precipitated by the same sera.