Porcine epsilon-trypsin is a three-chain inactivated trypsin from the limited autolysis of porcine beta-trypsin. It is cleaved at positions Lys60-Ser61 and Lys145-Ser146. The crystal structure has been determined by using the molecular replacement method, and refined at 1.8 A resolution. The R-value of final model is 0.184. Comparison with the electron density map of porcine beta-trypsin (PTRY) in complex (BBIT), and with that of native bovine beta-trypsin (HTNA), revealed no obvious changes except at the autolysis positions, and no changes at the active center were observed. The autolysis at positions Lys60-Ser61 and Lys145-Ser146 does not affect the conformation of His-57 in the active center and therefore cannot explain for a loss in porcine epsilon-trypsin activity.