We have investigated the effect of a monoclonal antibody on influenza virus release and the cell surface expression of M2, comparing virus strains which were observed previously to be sensitive (A/Udorn) or resistant (A/WSN and A/Udorn variants) to growth inhibition by M2 antibody 14C2. Incubation of A/Udorn virus-infected cells in the presence of the inhibitory M2 antibody resulted in a significant reduction in the yield of virus, as measured by infectivity assays as well as by the release of purified virions. The release of A/Udorn virus was not inhibited by the presence of monovalent 14C2 Fab, in contrast to IgG, indicating that a bivalent structure is essential for 14C2 antibody-mediated viral growth restriction. The level of M2 surface expression in A/Udorn virus-infected MDCK cells was found to be reduced to approximately 60% of control levels in cells incubated with the 14C2 antibody. In contrast, M2 surface expression levels in A/WSN virus-infected cells were decreased by only approximately 5-15%, and A/WSN virus assembly appeared to be unaffected by the M2 antibody treatment. M2 antigen associated with cell membranes and virus particles was redistributed into clusters after M2 antibody treatment in infected cells. Incubation in the presence of the 14C2 antibody also reduced M2 surface expression by approximately 40-50% in cells infected with a recombinant vaccinia virus that expresses the M2 A/Udorn protein. These results demonstrate that M2 antibody reduces the level of influenza virus particle formation in a single cycle of infection and suggest that inhibition of A/Udorn virus replication by the 14C2 antibody is related to the reduced cell surface expression and redistribution of the M2 protein induced by the antibody treatment.