Atlantic salmon (Salmo salar, L), 180 g, were fed diets containing a crude trypsin inhibitor from soybean at five levels. The five levels resulted in trypsin inhibitor activities (TIA) corresponding to an inclusion of 0 to about 100% commercial soybean meal. Digestibility of protein and fat, weight gain and trypsin activity in intestinal content were significantly reduced with increasing dietary TIA. The response curves for nutrient digestibility and growth indicated that the Atlantic salmon were able to compensate for a certain TIA, equivalent to about 5 mg bovine trypsin inhibited per g feed. This compensation seemed to be due to increased trypsin secretion as suggested by the values for total content of trypsin in the pyloric caeca homogenate. At the highest level of TIA, the pancreatic capacity of trypsin synthesis seemed to be exhausted.