A simple method for the purification of anionic and cationic trypsinogen and trypsin from human pancreatic juice applying affinity chromatography on aprotinin coupled Sepharose is described together with the N-terminal amino acid sequences for both trypsinogens. In addition, enzyme-linked immunoabsorbent assay (ELISA) methods for the determination of anionic and cationic trypsin-like immunoreactivity (irAT and irCT) are described. Normal serum levels are 21.3 +/- 7.4 micrograms/l and 27.8 +/- 9.0 microgram/l for irAT and irCT respectively and the accuracy of these assays is 6-10%. In our population, the normal ratio between irCT and irAT in serum is 1.36 +/- 0.42. In normal serum trypsin-like immunoreactivity consists solely of trypsinogen. In acute pancreatitis there is an increase over normal of both irAT and irCT with a proportionally greater increase in irAT than irCT. Similar changes are also found in uremic patients.