We employed a radioimmunoassay for human cationic trypsin to define the time course for alterations in the molecular forms of this enzyme in plasma from patients with pancreatitis. Six patients developed acute pancreatitis as a complication of a known disorder [three, Reye's syndrome; two, hemolytic uremic syndrome (HUS); one, choledochal cyst]. The immunoreactive forms of cationic trypsin were determined by gel filtration of each plasma sample followed by radioimmunoassay of the column fractions. Early in the course of the disease, predominantly free trypsinogen was released into the circulation in five patients. In the three patients with Reye's syndrome, subsequent plasma samples showed, in addition to free trypsinogen, increasing amounts of immunoreactive trypsin complexed to alpha 2-macroglobulin and alpha 1-protease inhibitor. In contrast, subsequent samples from the two patients with HUS contained little or no inhibitor-bound trypsin. The remaining patient had intermediate concentrations of cationic trypsin complexed to these two circulating protease inhibitors. Five patients died and postmortem studies showed a striking correlation between the histological severity of acute pancreatic inflammation and the amount of immunoreactive trypsin complexed to alpha 2-macroglobulin and alpha 1-protease inhibitor. This preliminary study suggests that measurement of alpha 2-macroglobulin or alpha 1-protease inhibitor-bound trypsin may be a useful method of monitoring the progression and severity of disease in patients with acute pancreatitis. Characterization of serial changes in the forms of circulating pancreatic proteases may enhance our understanding of time-dependent pathophysiologic events, possibly leading to improved forms of specific therapy.