An overlay and a solid-phase assay are presented which allow the specific detection of 9-O-acetylated sialic acids on sialoglycoconjugates immobilized on microtiter plates, nitrocellulose or separated on thin-layer chromatograms. The assay takes advantage of two different biological properties of influenza C virus, its high-affinity binding to 9-O-acetylated sialic acids and its sialate 9-O-acetylesterase that is used for detection of bound virus with fluorogenic or chromogenic substrates. Though simple and rapid, the assay is highly sensitive with a detection limit of 65 fmol 9-O-acetylated sialic acid in 9-O-acetylated ganglioside GD1a. Influenza C virus is able to bind to a wide spectrum of sialoglycoconjugates like mucins, serum glycoproteins or gangliosides containing naturally or synthetically O-acetylated sialic acids. 9-O-Acetyl-N-glycoloylneuraminic acid can also function as a high-affinity receptor determinant for influenza C virus. While the acetyl ester at the 9 position is essential for virus binding in all cases, a 4-O-acetyl group is not recognized. In addition to alpha(2.3) or alpha(2.6) bonds, 9-O-acetyl-N-acetylneuraminic acid in alpha(2.8) linkage to N-acetylneuraminic acid is also functionally active.