Alpha₁‐antitrypsin is the most abundant circulating protease inhibitor. The severe Z deficiency allele (Glu342Lys) causes the protein to undergo a conformational transition and form ordered polymers that are retained within hepatocytes. This causes neonatal hepatitis, cirrhosis, and hepatocellular carcinoma. We have developed a conformation‐specific monoclonal antibody (2C1) that recognizes the pathological polymers formed by α₁‐antitrypsin. This antibody was used to characterize the Z variant and a novel shutter domain mutant (His334Asp; α₁‐antitrypsin King's) identified in a 6‐week‐old boy who presented with prolonged jaundice. His334Asp α₁‐antitrypsin rapidly forms polymers that accumulate within the endoplasmic reticulum and show delayed secretion when compared to the wild‐type M α₁‐antitrypsin. The 2C1 antibody recognizes polymers formed by Z and His334Asp α₁‐antitrypsin despite the mutations directing their effects on different parts of the protein. This antibody also recognized polymers formed by the Siiyama (Ser53Phe) and Brescia (Gly225Arg) mutants, which also mediate their effects on the shutter region of α₁‐antitrypsin. Conclusion: Z and shutter domain mutants of α₁‐antitrypsin form polymers with a shared epitope and so are likely to have a similar structure. HEPATOLOGY 2010