A proteolytic susceptibility index was proposed to evaluate the reaction performance of different food by-products with subtilisin. Whey, salmon muscle and feather keratine were hydrolyzed at the same peptide bond concentration at 50 degrees C and pH 8.0 with different subtilisin concentrations. The logarithmic equation P = 1/b In (abt+1) was fitted to estimate the kinetic constants a and b. The a/b ratio was proposed as a proteolytic susceptibility index, which was correlated with the chemical structure and conformation of protein sources in increasing order of feather keratin, salmon muscle and whey proteins with corresponding a/b values of 0.1, 8.2 and 14.4 mM(2)/min, respectively. The methodology proposed in this work can be used to evaluate the proteolytic susceptibility of different protein by-products to different proteases and to evaluate the changes in proteolytic susceptibility after treatment.