The method suggested includes incubation of trypsin with native blood serum and subsequent quantitative determination of trypsin, that did not react with inhibitor, by means of the antitryptic precipitating serum. The method is based on the principle of radial immunodiffusion. In native serum that was not subjected to dilution the antitryptic capacity is 210.0 +/- 27.3 microng/ml in people, 238.7 +/- 33.1 microng/ml in rabbits. A distinct dynamics of changes in the antitryptic capacity of blood serum is observed with experimental hypertrypsinemia.