Five monoclonal antibodies (J14, J9, Q5, K16, S16), directed to three distinct antigenic sites (A-1, A-2, B-1) on the haemagglutinin-esterase glycoprotein of influenza C virus, were analysed for their ability to inhibit the receptor-destroying enzyme (RDE) activity of the virus, utilizing various assay systems. The ability of influenza C virus to destroy the receptors on chicken erythrocytes was inhibited efficiently by the antibodies to site A-1 (J14, J9, Q5) but not by those to site A-2 (K16) and sit B-1 (S16). Of the three antibodies to site A-1, J14 showed the highest inhibitory activity. Antibodies to sites A-1 and A-2 inhibited the ability of RDE to inactivate the haemagglutination inhibition activity of rat serum inhibitors, but the highest activity was observed again with J14. Thus the RDE site of influenza C virus may be located closest to the epitope recognized by J14. The removal of O-acetyl groups from either 9-O-acetyl-N-acetylneuraminic acid or p-nitrophenylacetate, caused by the viral RDE, was not prevented at all by any of the monoclonal antibodies tested. Furthermore, none of several polyclonal antiviral sera prepared in different animal species was able to block the hydrolysis of these small substrates, raising the possibility that the catalytic site of influenza C viral RDE is antigenically silent.