pH dependence of fusion between VSV and liposomes. (A) Kinetics of VSV fusion at various pH values, at 20°C, a.u. (arbitrary units). (B) pH dependence of the extent of fusion at 20°C, obtained from kinetic curves, such as those shown in A. The increase in fluorescence was maximal at pH 6.05, and this value was defined as 100%. The slight increase in fluorescence observed at pH values above 6.5 is due to virus–membrane interactions rather than membrane fusion. Error bars indicate the standard deviation (for three experiments).