Abstract
Influenza A viruses with noncleaved hemagglutinin (HA) are not internalized after adsorption, since they can be removed and/or inactivated by washing the infected cells at pH 2.5, in contrast to virus with cleaved HA. If uptake of influenza viruses into endosomes is a presupposition for fusion between viral and cellular membranes, this uptake does not occur with virus possessing noncleaved HA.