A model for evaluating lactose hydrolysis with immobilized enzyme in a packed bed reactor was developed. It is assumed that the conditions were plug flow and Michaelis-Menten kinetics with product competitive inhibition. The effects on the reaction rate of the external mass transfer resistance were considered by estimating the total fluid mass transfer coefficient. Experimental results were determined using β-galactosidase of Kluyveromices fragilis immobilized on chitosan beads in a packed bed reactor under special conditions, in order to calculate the parameters for different flow ranges and substrate concentrations. The model with these values allowed us to fit the reactor behavior for a wider range of work.