In gene libraries of Mycobacterium bovis BCG, Mycobacterium tuberculosis and Mycobacterium leprae recombinants were frequently found expressing an immunodominant 65 kDa protein antigen. In this study polyclonal and monoclonal antibodies against the 65 kDa antigen were found to react with a variety of different bacteria. Furthermore it is shown that the 65 kDa mycobacterial protein belongs to the family of antigens previously designated 'common antigen' due to their presence in a large variety of bacterial species. The molecular weight of this common antigen in 17 bacterial species was determined and it varied from 59 to 65 kDa. These bacteria included Gram-negative, Gram-positive and archae-bacteria, indicating that this antigen consists of members of an evolutionary well-conserved protein family. The 65 kDa protein was located in the cytoplasmic fraction of both Escherichia coli K12 and M. bovis BCG. Its function for the bacterial cell is presently unknown. The immunological relatedness of this common antigen to the MbaA protein might indicate a role in the etiology of rheumatoid arthritis, as was recently suggested for the mycobacterial 65 kDa antigen.