Mycobacterium tuberculosis CDC 1551, a highly immunogenic outbreak strain, previously reported to have unique surface distribution of capsular polysaccharide, was used to generate novel monoclonal antibodies (mabs) to surface mycobacterial targets. Two immunoglobulin G1 (IgG1) mAbs, 16a1 and 16a6 were generated. The mAbs originated from the same B cell, bound strongly to whole cell M. tuberculosis CDC1551 and to its cell wall, membrane and cytosol fractions recognizing a 90kDa protein. Immunoprecipitation using mAb 16a1 isolated a protein with amino acid peptide sequences matching MPT51 from the cytosol. This immunogenic protein of unknown function was previously reported only in culture filtrates of M. tuberculosis. Our findings suggest for the first time that this protein is found within the M. tuberculosis cell.