Determination of esterase activity of trypsin in blood serum using the Erlanger's method leads to overestimation of enzyme activity due to the increased "dimness" of the reaction medium. V. A. Shaternikov suggested a correction, which is based on the use of the two-wavelength registration method. But this correction does not eliminate this secondary factor completely. So for the clearing of the solutions we offer to stop the reaction by putting tubes into ice followed by subsequent centrifugation the investigated samples at 0 degree C for 15 min, at 25,000 g. It gives the possibility to measure an actual increase of an optical density in accordance to the hydrolysis of the substrate. The results of the measuring of an esterase activity of trypsin are represented in a serum and in a peritoneal exudate of dogs with an acute experimental pancreatitis. The serum esterase activity of trypsin in a was not found in intact animals. Three hours after induction of pancreatitis trypsin activity was detected only in blood serum of two of seven animals [approximately 3.6 mmole/(min-l)]. Rats with acute pancreatitis also had very low esterase activity of trypsin in blood [0.38 +/- 0.16 mmole/(min-l)] that insignificantly deferred from the control. These data question applicability of this method for diagnostics of acute pancreatitis. The estarse activity of trypsin detected in blood and peritoneal exudate, does not indicate its proteolytic activity, because it was insensitive to soybean trypsin inhibitor.