The 65 kD heat shock protein (HSP) has been implicated in the aetiology of recurrent aphthous stomatitis (RAS). We have previously demonstrated that peptide 91-105 derived from the sequence of mycobacterial 65 kD HSP stimulates specifically lymphocytes from patients with RAS. In this investigation, we show that both CD4+ and CD8+ T cells were significantly stimulated with mycobacterial peptide 91-105. In contrast, the human homologous peptide 116-130 stimulated only CD4+ T cells. Inhibition studies showed that CD4+ T cells were class II restricted, whereas CD8+ T cells were class I restricted. We then used truncated or substituted peptides, and demonstrated that residues 95-105 appear to be important, and residue 104(Arg) critical, in stimulating the T cells. Thus, peptide 95- 105 may constitute a T-cell proliferative epitope in RAS. We postulate that the high load of micro-organisms that colonize the oral mucosa may initiate an immune response by the microbial HSP 65-derived peptide 95-105, stimulating the numerous Langerhans cells in the oral mucosa to activate a cross-reacting immune response to the homologous peptide 116-130 within the epithelial HSP 60, initiating the immunopathological changes that lead to RAS.