The 65 kD protein of mycobacteria is an immunodominant antigen for both T and B lymphocytes. Sequence analysis has revealed that this protein belongs to the highly conserved family of stress proteins, related to the GroEL gene product of E. coli, that are present in all cells from bacteria to man. We demonstrate here that human T cells from healthy individuals and disease sites are able to recognize determinants within the 65 kD protein that are either specific for M. tuberculosis or are conserved between GroEL of mycobacterial, E. coli or human origin. The induction of T cells that recognize with cross-reactive sequences of GroEL may provide an explanation for the autoimmune phenomena often associated with infection by microbial pathogens. However, both the magnitude and the biological significance of this component of the T-cell repertoire reactive with self stress proteins will be influenced by local environmental factors as well as the MHC haplotype of the individual.