The neuraminidase inhibitor 2-deoxy-2,3-dehydro-N-trifluoroacetylneuraminic acid (FANA) inhibits the mutlicycle replication of influenza viruses in tissue culture. Influenza virus grown in the presence of FANA contains neuraminic acid on its envelope which then serves as receptor for other virus particles causing extensive aggregation. Thus, FANA inhibits influenza virus replication by preventing the enzymatic removal of neuraminic acid from the virus envelope.