The serological hallmark of primary biliary cirrhosis (PBC) is the presence of pyruvate dehydrogenase complex E2 subunit (PDC-E2) antimitochondrial antibodies (AMAs). Anti- PDC-E2 antibodies cross-react specifically with mycobacterial hsp65, and we have demonstrated that the motif SxGDL[ILV]- AE shared by PDC- E2212- 226 and hsp’s is a cross-reactive target. Having found that this same motif is present only in β -galactosidase of Lactobacillus delbrueckii (BGAL LACDE), we hypothesized that this homology would also lead to cross-reactivity. The mimics were tested via ELISA for reactivity and competitive cross-reactivity using sera from100 AMA-positive and 23 AMA-negative PBC patients and 190 controls. An Escherichia colt (ECOLI) PDC-E2 mimic that has been pathogenetically linked to PBC but lacks this motif has been also tested. Anti-BGAL266-280 LACDE antibodies were restricted to AMA-positive patients (54 of 95, 57% ) and belonged to immunoglobulin (Ig) G3. Of the 190 controls, 22 (12% ; P < .001) had anti-BGAL266-280 antibodies, mainly of the IgG4 subclass. ECOLI PDC-E2 reactivity was virtually absent. BGAL266- 280/PDC- E2212- 226 reactivity of the IgG3 isotype was found in 52 (52% ) AMA-positive PBC patients but in only 1 of the controls (P < .001). LACDE BGAL266- 280/PDCXE2212- 226 reactivity was due to cross-reactivity as confirmed via competition ELISA.Antibody affinity for BGAL266- 280 was greater than for PDC- E2 mimics. Preincubation of a multireactive serum with BGAL266- 280 reduced the inhibition of enzymatic activity by 40% , while marginal effect (12% ) or no effect (2% ) was observed in human or ECOLIPDC-E2 mimics. In conclusion, IgG3 antibodies to BGAL LACDE cross-react with the major mitochondrial autoepitope and are characteristic of PBC. The serological hallmark of primary biliary cirrhosis (PBC) is the presence of pyruvate dehydrogenase complex E2 subunit (PDC-E2) antimitochondrial antibodies (AMAs). Anti- PDC-E2 antibodies cross-react specifically with mycobacterial hsp65, and we have demonstrated that the motif SxGDL [ILV] - AE shared by PDC- E2212- 226 and hsp’s is a cross-reactive target. Having found that this same motif is present only in β-galactosidase of Lactobacillus delbrueckii (BGAL LACDE), we hypothesized that this homology The mimics were tested via ELISA for reactivity and competitive cross-reactivity using sera from 100 AMA-positive and 23 AMA-negative PBC patients and 190 controls. An Escherichia colt (ECOLI) PDC-E2 mimic that has been Anti-BGAL266-280 LACDE antibodies were restricted to AMA-positive patients (54 of 95, 57%) and belonged to immunoglobulin (Ig) G3. Of the 190 controls, 22 (12% (P <.001) had anti-BGAL266-280 antibodies, mainly of the IgG4 subclass. ECOLI PDC-E2 reactivity was virtually absent. BGAL266- 280 / PDC- E2212- 226 reactivity of the IgG3 isotype was found in 52 ) AMA-positive PBC patients but only 1 of the controls (P <.001). LACDE BGAL266-280 / PDCXE2212- 226 reactivity was due to cross-reactivity as confirmed via competition ELISA. Antibody affinity for BGAL266-280 was greater than for PDC-E2 mimics. Preincubation of a multireactive serum with BGAL266- 280 reduced the inhibition of enzymatic activity by 40%, while marginal effect (12%) or no effect (2%) was observed in human or ECOLIPDC- E2 mimics. In conclusion, IgG3 antibodies to BGAL LACDE cross-react with the major mitochondrial autoepitope and are characteristic of PBC.