The pathogenesis of endoscopic retrograde cholangiopancreatography (ERCP)-induced pancreatitis is poorly understood. To elucidate a role for pancreatic enzymes in ERCP-induced pancreatitis, we measured serum amylase, lipase, trypsin, and elastase in 25 patients undergoing ERCP. Serum alpha 1-antitrypsin and alpha 2-macroglobulin, two major pancreatic protease inhibitors, also were measured. All pancreatic enzymes measured rose significantly after ERCP. Pancreatic duct cannulation was associated with a greater elevation in serum amylase and lipase. Circulating alpha 2-macroglobulin was reduced by 7% (p = 0.04) 6 h after ERCP, whereas circulating alpha 1-antitrypsin increased over the same time period. Papillotomy, stent placement, or underlying disease did not influence changes any further. Three patients developed ERCP-induced pancreatitis. All three patients had circulating alpha 2-macroglobulin levels below 243 mg/dl (p = 0.03). The ERCP-induced alterations in circulating pancreatic enzymes and their inhibitors are similar to changes seen in clinical pancreatitis. Low circulating alpha 2-macroglobulin levels may predispose to ERCP-induced pancreatitis.