The research field for applications for lactose hydrolysis has been investigated for some decades. Lactose intolerance, improvement for technical processing of solutions containing lactose and utilisation of lactose in whey are main topics in development of biotechnological processes. In this article, the establishment of a hollow fiber membrane reactor process for enzymatic lactose hydrolysis is reported. Mesophilic beta-galactosidases were circulated abluminally during luminal flow of skim milk. The main problem, microorganisms growth in the enzyme solution, was minimised by sterile filtration and UV irradiation. In order to characterise the process parameters, such as skim milk concentration, enzyme activity and flow rates were varied. In comparison to a batch process, enzyme activity could be used longer and enzyme rest into the product should not occur. Furthermore, the three-dimensional separation of the substrate from the enzyme solution minimise blocking and washing out effects, which restrict processes with immobilised enzymes. A conversion rate of 78.11% was achieved at a skim milk flow rate of 9.9l h(-1), enzyme activity of 120 Uml(-1) and a temperature of 23+/-2 degrees C in a hollow fiber reactor with a membrane area of 4.9 m2.