The molecular locations of antibody binding sites on the haemagglutinin of influenza virus X-31 were investigated by electron microscopy of haemagglutinin-monoclonal antibody complexes. Evidence was obtained for different sites of binding of different antibodies and direct correspondence was observed between these sites and the locations of antigenic sites A, B, and E (D. C. Wiley, I. A. Wilson, and J. J. Skehel (1981). Nature (London) 289, 373-378) defined by determining the amino acids recognized by the specific antibodies.