Allantoic fluids harvested from embryonated chicken eggs infected with reference strains of influenza A viruses were analysed for subtilisin inhibitor activity. While all acid heat-treated and nontreated virus-infected fluids could reduce subtilisin activity, fluids of FM and Bangkok strains had the greatest inhibitory ability. The degree of subtilisin inhibition closely paralleled the appearance of infectious Bangkok and FM virus in allantoic fluid. Maximum levels were achieved at 48 hr post-infection (p.i.) Ultracentrifugation analyses indicated that the bulk of thermostable inhibitor(s) of 48 hr Bangkok and FM infectious fluids remained in the supernatant rather than sedimenting with the viral pellet.