The influenza virus haemagglutinin (HA) glycoprotein is being studied both as a prototype membrane surface molecule and as a functional component of a viral envelope. X-ray crystallographic studies of the HA from the human, 1968 Hong Kong virus strain produced atomic models from which chemically detailed conclusions about virus functions can be deduced. The detail of these models also permits the design of biochemical, recombinant DNA, and cell biological experiments on the mechanisms of the viral activities. A long-term goal is to develop strategies to intervene in the viral processes to prevent infection. How the virus binds to cells is being studied by X-ray structural analysis of complexes between the HA and synthetic receptor analogs and with complexes between sialocides and affinity mutants of the HA. The structure of the HA from an animal influenza virus Duck/Ukraine/63 the progenitor of the 1968 human Hong Kong strain will be determined. The energetics of the receptor binding interactions are being measured in parallel with X-ray experiments by NMR. Recombinant DNA methods are being used to create site-directed mutations in the interacting residues and to provide quantities of novel molecular species for NMR and X- ray studies. The mechanism of membrane-fusion by which many enveloped animal virus enter cells is being studied by structural analysis of a fusion active and precursor, inactive form of the HA. The structure of membrane fusion mutants and a second fusion active glycoprotein from Influenza C virus are also being studied.