Abstract
Antiserum to pure M-protein extracted from PR8 virions neutralized the infectivity and inhibited the haemagglutinating activity of various influenza A virions. It agglutinated concentrated suspensions of these virions and fixed complement in their presence. Antibodies to M-protein were readily absorbed by intact virions or by spikeless particles obtained after proteolytic treatment, giving clear evidence that M- protein is exposed at the surface of the virus envelope. The data suggest that when antibodies to M-protein occupy specific ligands exposed at the surface of the virion they interfere with sites critical for infectivity and haemagglutinating activity.