Antimitochondrial antibodies (AMA) directed against the lipoyl domain of the E2 subunit of pyruvate dehydrogenase (PDC-E2) are detected in 95% of patients with PBC and are present before onset of clinical disease. The recent demonstration that AMA recognize xenobiotic modified PDC-E2 with higher titers than native PDC-E2, raises the possibility that the earliest events involved in loss of tolerance are related to xenobiotic modification. We hypothesized that reactivity to such xenobiotics would be predominantly IgM and using sera from a large cohort of PBC patients and controls (n=516), we examined in detail sera reactivity against either SAc-conjugated bovine serum albumin (BSA), recombinant PDC-E2 (rPDC-E2) or BSA alone. Further, we also defined the relative specificity to the SAc moiety using inhibition ELISA; SAc conjugate and rPDC-E2 specific affinity purified antibodies were also examined for antigen specificity, isotype and cross-reactivity. Reactivity to SAc conjugates is predominantly IgM; such reactivity reflects a footprint of previous xenobiotic exposure. Indeed, this observation is supported by both direct binding, cross reactivity, and inhibition studies. In both early and late stage PBC, the predominant Ig isotype to SAc is IgM, with titers higher with advanced stage disease. We also note that there was a higher level of IgM reactivity to SAc in early stage versus late stage PBC. Interestingly, this finding is particularly significant in light of the structural similarity between SAc and the reduced form of lipoic acid, a step which is similar to the normal physiological oxidation of lipoic acid. We submit that specific modifications of the disulfide bond within the lipoic-acid-conjugated PDC-E2 moiety, i.e. by an electrophilic agent renders PDC-E2 immunogenic in a genetically susceptible host.