Summary Antibody directed against the neuraminidase of an influenza virus did not prevent attachment of the virus to red cells, but strongly inhibited elution of the virus from these cells. The virus used in these experiments had approximately 500 enzyme antigenic sites on the surface of each particle. In the presence of sufficient antibody to combine with all of these sites there was no measurable elution of virus from cells. In the presence of smaller amounts of antibody, the virus eluted slowly.