A modified version of the human pancreatic trypsin inhibitor (PSTI), generated in a protein-design project, has been crystallized in spacegroup P4(3) with lattice constants a = 40.15 A, c = 33.91 A. The structure has been solved by molecular replacement. Refinement of the structure by simulated annealing and conventional restrained least-squares yielded for 8.0 to 2.3 A data a final R-value of 19.1%. Differences to the known structures of porcine PSTI complexed with trypsinogen and modified human PSTI complexed with chymotrypsinogen occur at the flexible N-terminal part of the molecule. These differences are influenced by crystal packing, as are low temperature factors for the binding loop. The geometry of the binding loop is similar to the complexed structures.