A panel of nine monoclonal antibodies to Mycobacterium leprae were used to characterize a protein antigen of the bacillus. Two monoclonal antibodies (IVD8 and IIIE9) were specific for M. leprae and reacted with an epitope (CWPa) present on a protein molecule associated with the cell wall fraction of M. leprae. This protein, designated cell wall-associated protein (CWP), lost its immunoreactivity upon treatment with trypsin and had an apparent molecular weight of 65,000, though additional lower-molecular-weight forms of the protein were observed by immunoblotting. Four other cross-reactive epitopes (CWPb, CWPc, CWPd, and CWPe) were defined on the same molecule using seven independent monoclonal antibodies. Therefore, M. leprae possesses a trypsin-sensitive, heat-stable protein associated with the cell wall which contains at least one species-specific and four cross-reactive antigenic determinants.