Fusion of influenza virus with liposomes is triggered by low pH, resulting in a conformational change in the fusion protein (HA) and the insertion of fusion peptides from HA into the liposomal membrane. Fusion does not take place immediately after insertion but is preceded by a lag phase, the duration of which, as we have found previously, depends on the presence of ganglioside receptors in the liposomal membrane [Stegmann, T., White, J. M., & Helenius, A. (1990) EMBO J. 9, 4231-4241]. Here we have investigated why that is the case. Surprisingly, the 2-4-fold shorter lag phase observed with phosphatidylcholine (PC)/phosphatidylethanolamine (PE)/ganglioside liposomes was not due to slower or more readily reversible binding of the virus to PC/PE liposomes lacking receptors. Nevertheless, using liposomes with various glycolipids as targets, it was found that specific HA-receptor interactions were required for a shorter lag, and not just the negative charge of the gangliosides, or the presence of ceramide lipid tails in the liposomal membrane. Receptor binding also did not facilitate the conformational change in HA. Surprisingly, however, it was found that after an incubation of the virus at low pH in the absence of target membranes at 0 degrees C for several minutes, the binding and fusion activity of virus using PC/PE liposomes, but not PC/PE/ganglioside liposomes as targets, was decreased. The population of virus that did still bind to and fuse with the PC/PE liposomes after low pH preincubation did so after a significantly increased lag time.(ABSTRACT TRUNCATED AT 250 WORDS)