Extracellular cleavage of virus envelope fusionglycoproteins by host cellular proteases is a prerequisite for the infectivity of mammalian and nonpathogenic avian influenza viruses, and Sendai virus. In search of such target processing pro-teases in the airway, we recently found a new candidate trypsin-like processing protease in rat lungs, which was induced by Sendai virus infection, and identified as ectopic rat anionic trypsin I.