The hemagglutinin (HA) spike glycoprotein of influenza virus catalyzes a low pH-induced membrane fusion event which releases the viral genome into the host cell cytoplasm. To study the fusion mechanism in more detail, we have prepared the ectodomain of HA in water-soluble form by treating virus particles with bromelain. Under mildly acidic conditions (pH less than or equal to 5.8), the ectodomain undergoes a conformational change which we found to be biochemically and immunologically equivalent to that in native viral HA. It became sensitive to proteinase K, it exposed new antigenic epitopes in its HA1 chain, and it acquired amphiphilic properties, notably the ability to bind to liposomes. The attachment to liposomes exhibited the same pH dependence and rapid kinetics as the conformational change and was mediated by HA2. The nature of the attachment resembled that of an integral membrane protein except that the bound HA was partially removed by base. As observed for virus fusion, attachment is independent of divalent cations and lipid composition. Temperature was found to be a critical parameter only with dimyristoylphosphatidycholine vesicles where attachment was partially blocked below the major phase transition. These and other results obtained indicated that the low pH-induced conformational change in the isolated ectodomain is equivalent to that occurring in intact viral HA, and that its attachment to liposomes can serve as a model for the initial stages in the HA-induced membrane fusion reaction.