A single radial immunodiffusion method is described for determining the serum protein which reacts with antibody against the urinary trypsin inhibitor, but does not react with anti-inter-alpha-trypsin inhibitor antibody. Since the amount of this protein could not be determined with an agar plate containing antibody alone, we first prepared an agar plate containing 4% polyethylene glycol 6 000 and 1 mg of anti-urinary trypsin inhibitor gamma-globulin and confirmed that the amount of this protein could be measured accurately from the sizes of precipitin rings after 72 h incubation at room temperature. Levels of this serum protein, alpha 1-antitrypsin, alpha 2-macroglobulin and inter-alpha-trypsin inhibitor were measured in normal human serum by the single radial immunodiffusion method, and it was confirmed that the level of this serum protein did not correlate with the levels of these 3 other proteins.