Adjuvant arthritis (AA) can be induced in genetically susceptible rats by immunization with heat-killed mycobacteria suspended in mineral oil. From our analysis of arthritogenic T cell clone A2b, obtained from an arthritic Lewis rat and specific for the 180-188 epitope of mycobacterial 65-kDa heat-shock protein (hsp 65), the possible origin of AA was explained by the existence of a molecular mimicry of the 180-188 epitope with a cartilage-associated self antigen. We now have shown that Lewis rats respond to the 180-188 epitope after Mycobacterium tuberculosis immunization and that arthritis-resistant Fisher and (Lewis x Fisher)F1 rats, although major histocompatibility complex class II identical with Lewis, do not respond to this epitope. However, in rare cases of arthritis in Fisher rats, responses to the epitope were seen. We obtained no evidence for a defect at the level of antigen processing and presentation or for suppression in Fisher rats. Thus, non-responsiveness in Fisher rats was likely due to a difference at the level of the T cell repertoire. Previously, we have reported that pretreatment with hsp 65 in experimental arthritis, and not only in AA, caused resistance to arthritis induction. We now present evidence that immunization with hsp 65 or in vitro stimulation with hsp 65 may lead to inhibition of responses specific for epitope 180-188. Thus the hsp 65-induced resistance to arthritis is probably caused by the induction of regulatory control specifically targeted at the 180-188 epitope. Especially in rats that tend to focus their responses on the critical 180-188 sequence, such as Lewis, regulation seems to develop following immunization with hsp 65. Since recent evidence suggests that hsp 65 and also the 180-188 epitope have a role in human arthritic conditions, the present findings are expected to contribute to further experimentation directed at exploiting hsp 65 or its epitopes for the development of new therapeutical approaches in humans.