Horseradish peroxidase (HRP) was immobilized onto hydrophobic polypropylene microfiltration membranes (PPMMs) through physical adsorption. Effects of incubation time and initial HRP concentration were tested and the most appropriate condition was determined to be 24 h of incubation in 0.3 mg/mL HRP solution. However, the hydrophobicity of the PPMM can be a source of protein denaturation. Then, four surface modifications were conducted to enhance the hydrophilic character of the support and thus the efficiency of HRP immobilization. Results revealed that surface modification with functional groups such as glucoside, polypeptide or amino groups improved the enzyme stability and particularly allylamine plasma treatment led to the highest membrane activity. Then two more methods were used to link HRP onto allylamine plasma treated PPMM (AAMP-PP), such as glutaraldehyde, linkage via reaction with amino groups and molecular recognition via biotin-avidin interaction. The results demonstrated that more HRP could be immobilized onto AAMP-PP membranes. In addition, higher membrane activity was observed when the HRP was fixed through chemical reaction and almost 60 times larger when the HRP was attached by molecular recognition. The storage stability of immobilized HRP through biotin-avidin recognition was much higher than those through adsorption or glutaraldehyde binding, almost similar to the free HRP in buffer solution. This means that the immobilization route was to some extent a parameter of more importance than the membrane surface treatment and presumably that biotin-avidin method on allylamine plasma treated PPMM provided a more favorite environment for HRP molecules to keep their natural conformation holding most of their activity. (c) 2006 Elsevier Inc. All rights reserved.