Renal clearances of amylase isoenzymes, expressed as percentages of creatinine clearances, were determined in 20 normal subjects and in 8 patients with acute pancreatitis. The isoenzyme assay employed thin layer isoelectric focusing, starch iodine staining, and densitometry. Normal clearance of pancreatic-like amylase (mean +/- SE: 3.00 +/- 0.40%) was greater than the clearance of salivary-like amylase (0.51 +/- 0.06%) in ea ch individual (P less than 0.001). However, the amount of pancreatic amylase in the serum was not the major determinant of amylase clearance. Normal clearance of pancreatic-like amylase was significantly (P less than 0.001) less than the clearance of total serum amylase in acute pancreatitis (6.49 +/- 1.07%). In pancreatitis the clearance of pancreatic-like amylase (7.29 +/- 1.19%) and the clearance of salivary-like amylase (4.55 +/- 1.02%) were both elevated compared to normal (P less than 0.001). These findings indicate that the increased clearance of amylase in pancreatitis results from a change in renal function rather than a change in serum amylase. Renal changes not reflected by increased serum creatinine or more than mild proteinuria may be manifestations of the systemic toxicity of acute pancreatitis.