Immunization of rabbits with Kunitz soybean trypsin inhibitor results in the formation of antisera that can be utilized for the detection and quantitative determination of this protein. The trypsin inhibitor was isolated from a commercial preparation by isoelectric focusing in the pH region between 3 and 6. The inhibitor was studied by several immunochemical methods including double gel diffusion, immunoelectrophoresis, immunoelectrofocusing, quantitative precipitation, single diffusion, and radial immunodiffusion. The quantitative aspects of some of these methods are discussed. A water-insoluble polymer of the trypsin inhibitor obtained by the reaction of the protein with ethyl chloroformate was used as an immunoadsorbent for the purification of the antitrypsin inhibitor antibody. © 1969.