In vitro protein solubility at pH 3, 6 and 9 in various feedstuffs was determined, by measuring the amount of crude protein (6.25 x N) passing a paper filter after 2 h incubation at 40°C. Also, the enzymatic hydrolysis of the insoluble proteins was determined after incubation (2h, 40°C) with either pepsin, neutrase (protease of Bacillus subtilis) or alcalase (endoproteinase of Bacillus licheniformis), which are proteolytic enzymes with pH optimum of approximately 3, 6 and 9, respectively. The feedstuffs tested showed a great variation in protein solubility and hydrolysis. Pea and bean samples had a high protein solubility at pH 9 (78 to 85%), which was decisively lower at pH 3 (0 to 28%). On the contrary, for other feedstuffs, such as maize by-products and meat-and-bone meal, the differences in protein solubility at different pH was less pronounced. The enzymatic hydrolysis of pea and bean proteins with alcalase at pH 9 ranged between 0 and 15%, while in most other feedstuffs these values were considerably higher. Neutrase activity (pH 6) was poor in all feedstuffs with the exception of the wheat products. It was concluded that the origin of the proteins had great influence on the variation in protein solubility at different pH and hydrolysis with enzymes with different pH-optimum.