Neutral protease activity of allantoic fluid from embryonated chicken eggs was quantified during the course of influenza virus infection. Antigenic subtypes of influenza A viruses selected for study were H1N1 strains PR/8/34, Brazil/8/78, FM/1/47, the H3N2 strain Bangkok/1/80 and the H5N9 Turkey/ /Ontario/66 as well as the Sendai strain of parainfluenza type 1 virus. Three different types of profiles of allantoic fluid proteases could be readily distinguished after infection of eggs with various virus strains. In all profiles, periodic peaks of protease activity always preceded the partial shut down of protamine cleaving proteases which paralleled the production of near maximum titers of infectious virus. To determine the mechanism involved in this reduction of proteolytic activity, infectious allantoic fluids were analysed for the presence of protease inhibitors. Acid heat treated 48 hour virus-infected allantoic fluids of different influenza strains could inhibit the activities of subtilisin and allantoic fluid proteolytic enzymes.