The active trypsin inhibiting component, SPC1, was obtained during the synthesis of a 22-residue peptide with three disulfide bridges according to the mimic mung bean Bowman-Birk type inhibitor. The K(i) value of SPC1 is 1.2x10(-7) M. In order to determine the topological structure of SPC1, X-ray diffraction studies were carried out on the complex of SPC1 with bovine beta-trypsin. Only the binding loop of SPC1 resolved at 2.2 A resolution due to conformational flexibility of the other residues [1]. The amino acid sequence was re-determined and electrospray mass spectroscopy was also performed to ensure that no cleaving occurred on SPC1 and the primary sequence of SPC1 is correct. Because the protein is more rigid in nonaqueous medium as has been proved by others [2], we treated the complex of SPC1 with neat cyclohexane and then subjected it to X-ray diffraction analysis, and the result showed that all the 22 residues of SPC1 were located in the electron density map. So the topological structure of SPC1 has been determined, suggesting that crystal treatment with cyclohexane may be used as a method to determine the conformation of the disordered regions in protein crystal structures.