Enzymic hydrolyses of naturally occurring protein sources represent complex reactions at a molecular level because of the specificity pattern of proteases and the composition of the substrates involved. In addition, rather global criteria are in practical use for describing the extent of protein hydrolysis. Therefore, mechanistically based kinetic modelling may not be applicable under these circumstances. For practical purposes it is often sufficient to use a descriptive mathematical model, which leads to a reliable interpolation of the experimental data. Empirical models are proposed for the kinetics of the limited tryptic whey-protein hydrolysis. These models are identified by fitting profiles of the fraction of soluble protein (non-protein nitrogen, NPN) as a function of operating time of discontinously operated stirred-tank reactors. Models with at least four parameters are required in order to describe profiles obtained at temperatures at which strong enzyme inactivation is observed. A simple model of only two parameters based on a kinetics with exponential decrease of the activity with increasing fraction of soluble protein is sufficient to simulate the tryptic digestion of whey-protein in batch reactors under conditions of moderate enzyme inactivation applicable for temperatures lower than 60 degrees C.