Novel trypsin inhibitors from the white rot fungus Abortiporus biennis were isolated, partially purified, and characterized. The inhibitors were purified by heat treatment, anion-exchange chromatography, and gel filtration. SDS-PAGE of the purified preparation demonstrated the presence of two proteins with molecular masses of 20 and 21.5 kDa. The A. biennis inhibitors were most active against trypsin, while chymotrypsin alpha, proteinase K, and Carlsberg subtilisin were inhibited to a smaller extent. The inhibitors are acidic proteins with remarkably high heat stability.