Fasting serum concentrations of trypsin and amylase activity have been compared in 107 subjects, including 18 controls and patients with mumps, acute pancreatitis, chronic pancreatitis, cancer of the pancreas, and chronic renal failure. There was no significant correlation between amylase activity and trypsin concentrations in any of these groups. In all 12 patients with acute pancreatitis and all 16 with chronic renal failure the serum immuno-reactive trypsin concentrations were elevated. Amylase activity was increased in 87% (20 out of 23) of patients with mumps, but only 13% (3 out of 23) had hypertrypsinaemia suggesting subclinical pancreatitis. In 18 patients with chronic pancreatitis low levels of serum trypsin were measured in 11 (61%), reflecting a decrease in pancreatic acinar mass. In contrast, serum amylase was normal or raised in all 18. Subnormal values of the trypsin to amylase ratio was obtained in 15 (83%). Trypsin levels in 20 patients with carcinoma of the pancreas were abnormal in 11 (55%). Six (30%) had abnormal amylase levels. It is concluded that it is more useful to measure the serum trypsin concentration than the amylase activity in the diagnosis of both mumps-pancreatitis and chronic pancreatic disease and that the trypsin to amylase ratio is more sensitive than either enzyme alone in the diagnosis of chronic pancreatitis.