Neutrase(R), used for hydrolysis of wheat proteins, was inhibited by end-product in a competitive uncompetitive way. The inhibition ratio depends on the progress of protein hydrolysis (degree of hydrolysis) and it remains constant beyond a degree of hydrolysis of 7.5%. The inhibitor was separated, on Sephadex G-25 column, in three fractions (>2.4, 2.4-0.5 and <0.5 kDa) generating an inhibition ratio of 29%, 46% and 67% respectively. The peptides size distribution (<1, 1-2, 2-3 and >3 kDa) of fractions was determined using size exclusion-high performance liquid chromatography. The analysis of obtained data, using a simple mathematical regression, showed a correlation factor of 0.98 between the inhibition ratio and the peptides less than I kDa and 0.99 when considering the peptides lower than I kDa and higher than 3 kDa. (C) 2003 Elsevier Ltd. All rights reserved.