High nitrogen barley syrup (HNBS) was obtained by mashing barley spent grains with neutral proteinase. The total nitrogen to carbohydrate ratio in the HNBS was three times higher than in typical barley syrup. In order to enhance the liberation of free amino acids, the spent grains were hydrolysed simultaneously with proteinase and barley peptidase extract; the alpha-amino to total nitrogen ratio increased from 11.4 to 14.6%. During mashing with peptidase extract, the protein solubilization rate was lower than during mashing with neutral proteinase only. Data analysis by means of the modified Foster-Niemann equation suggested that endopeptidase inhibitors, possibly present in the peptidase extract, caused the decrease of protein solubilization rate. The isolation of the bacterial proteinase inhibitors from peptidase extract indicates that they affected the protein solubilization process.