Eighteen derivatives of 2-deoxy-2,3-dehydro-N-acetylneuraminic acid were assayed for inhibitory activity against neuraminidases from viral and bacterial sources. Twelve of these compounds were active against neuraminidases of Vibrio cholerae, influenza A Mel, B Lee, and Newcastle disease virus, causing 50% enzyme inhibition in concentrations ranging from 10-3 M to 10-6 M. The most active of them and the most potent neuraminidase inhibitor described so far is 2-deoxy-2,3-dehydro-N-trifluoroacetylneuraminic acid. This compound has an inhibitor constant (Ki) of 7,9 × 10-7, M for influenza A Mel virus neuraminidase whereas the Km of the virus enzyme for the substrate is 1000 times weaker (Km = 6, 9 × 10-4 M). The mechanism of inhibition is competitive, and enzyme inhibition is independent of enzyme concentration. 2-Deoxy-2,3-dehydro-N-trifluoroacetylneuraminic acid inhibits hemagglutination by NDV and SV5 but does not inhibit agglutination of red cells by Sendai virus or influenza A and B viruses.