All work on human trypsinogen activation peptide (TAP) in acute pancreatitis has been carried out with the same assay. Despite the extensive use of this original TAP assay, there is no characterization of the TAP-like immunoreactivity measured.The aims of this study were to develop an additional TAP assay and to attempt to characterize the TAP-like immunoreactivity found in the urine of patients with acute pancreatitis.Antibodies against the human TAP were prepared using the whole octapeptide APFD4K, conjugated at its N-terminal end. Characterization of the immunoreactivity measured with these assays was performed using gel filtration of human pancreatic juice before and after activation of trypsinogen with enterokinase.After activation of the pancreatic juice, there was a large initial increase in immunoreactive TAP and a decrease 6-24 hours later. Using our antiserum, we found low levels of immunoreactive TAP in urine from patients with acute pancreatitis, although many of these samples contained high levels of immunoreactive TAP when tested with the commercially available TAP kit (Biotrin). The pentapeptide D4K, used as a standard in the Biotrin kit, showed much lower immunoreactivity than the synthetic octapeptide APFD4K in our assay. The octapeptide, however, reacted similarly to D4K in the Biotrin kit assay.n Our antibody prepared against the synthetic octapeptide APFD4K is directed against the N-terminal part of the octapeptide and does not recognize the pentapeptide D4K. Immunoreactive TAP in urine in acute pancreatitis is mainly composed of the C-terminal pentapeptide, D4K.