Amylase thermolability was determined by heating sera from control patients and patients with acute pancreatitis and pancreatic pseudocysts at 65 degrees C for 15 min. Normal serum amylase was 44 per cent thermolabile, but increased to 84 per cent thermolability during the hyperamylasaemia of acute pancreatitis, and returned to normal on resolution of the pancreatitis. In 9 patients with a pancreatic pseudocyst, although the total amylase level had almost returned to normal limits, 79 per cent of the amylase remained thermolabile. Amylase thermolability appears to be a simple screening test for pancreatic pseudocyst formation secondary to acute pancreatitis.