Increasing resistance to currently available influenza antivirals highlights the need to develop alternate approaches for the prevention and/or treatment of influenza. DAS181 (Fludase), a novel sialidase fusion protein that enzymatically removes sialic acids on respiratory epithelium, exhibits potent antiviral activity against influenza A and B viruses. Here, we use a mouse model to evaluate the efficacy of DAS181 treatment against a highly pathogenic avian influenza H5N1 virus. When used to treat mice daily beginning 1 day before infection with A/Vietnam/1203/2004(H5N1) virus, DAS181 treatment at 1 mg/kg/day protected 100% of mice from fatal disease, prevented viral dissemination to the brain, and effectively blocked infection in 70% of mice. DAS181 at 1 mg/kg/day was also effective therapeutically, conferring enhanced survival of H5N1 virus-challenged mice when treatment was begun 72 h after infection. This notable antiviral activity underscores the potential utility of DAS181 as a new class of drug that is effective against influenza viruses with pandemic potential. © 2007 by the Infectious Diseases Society of America. All rights reserved.