Three mathematical models, two logistic models (previously published in previous works) and one mechanistic, developed in this work and based on MichaelisMenten kinetics, were compared to select the most adequate model in describing the angiotensin-converting enzyme (ACE)-inhibitory activity of bioactive peptide mixtures obtained from cheese whey protein. The significance of both the model and its parameters as well as the value of the regression coefficient was used as criteria to select the most adequate model for obtaining the IC50 values corresponding to each bioactive peptides mixture. The best results were obtained with the MichaelisMenten-based model because it provided the best fits and in addition the values for its parameters were always significant. As parameters of this model have a physical meaning, it could be used for inhibition-testing experiments in the development of novel bioactive peptides. The results obtained indicated that the peptide mixture derived from the neutrase hydrolysis exhibited strong ACE inhibition activity. The main active peptides were short, with molecular masses below 1 kDa (IC50 = 40.37 +/- 2.66 mu g/mL) and represent 38% of the initial protein content in the hydrolysate. (c) 2012 American Institute of Chemical Engineers Biotechnol. Prog., 2012