SummaryThe hydrolytic and cost effi ciencies of fi ve endopeptidases (Alcalase 2.4L, Corolase 7089, Neutrase 0.8L, Promod 671L and Protex 7L) to hydrolyze Atlantic salmon by-products were compared at standardized activity levels based on a casein assay. The substrate was characterized prior to the hydrolytic experiments (pH=6.5, t=50 °C) to obtain substrate--specifi c constants for nitrogen to protein mass (in g) ratio, i.e. conversion factor f N =5.23 and total amount of peptide bonds (h tot )=9.3 mmol per g of protein.At low enzyme activity to substrate ratio, all enzymes were equally effi cient in hydrolyzing the substrate. At highest enzyme activity to substrate ratio, Protex 7L, Alcalase 2.4L and Promod 671L gave higher degree of hydrolysis (DH=14.2-14.6 %) than Corolase 7089 (13.2 %) and Neutrase 0.8L (11.6 %) aft er 120 min of hydrolysis. No diff erences were observed in protein recovery (yield of solubilized protein) relative to DH. Determination of DH was followed by the pH-STAT and o-phthaldialdehyde methods. Based on pH-STAT data, response surface regression models were established based on the combined eff ects of hydrolysis time and enzyme activity to substrate ratio on DH and protein recovery. The modelling approach was combined with enzyme cost to identify the most cost-effi cient enzyme (Protex 7L).