The role of neuraminidase and the mechanism of low pH dependence in influenza virus-induced membrane fusion have been studied further using fowl plague virus (FPV, H7N1). Two specific anti-FPV neuraminidase antisera obtained from chickens immunized with recombinant virus strains inhibited viral neuraminidase activity without influencing its haemagglutinating activity. These sera totally inhibited the FPV-induced fusion of erythrocytes and partially reduced haemolysis. But both fusion and haemolysis activities could be restored by external addition of Vibrio cholerae neuraminidase, indicating participation of neuraminidase in FPV-induced membrane fusion. With regard to low pH-dependent fusion by influenza virus, it was found that erythrocytes of various species showed different pH optima for haemolysis by FPV and that erythrocytes could be sensitized for fusion and haemolysis by FPV at neutral pH if they had been pretreated with a low pH buffer. These results demonstrated that surface properties of erythrocytes rather than that of the virus are critical in the low pH-dependent fusion and haemolysis by influenza viruses.