For influenza virus hemagglutinin, an N-cap structure, created at low pH, interacts with membrane-proximal residues (173-178), bringing fusion peptides and membrane-spanning domains close together. Mutational analysis was used to define the role of these interactions in membrane fusion. For all N-cap mutants, both lipid and aqueous dye spread was greatly reduced. Mutation at residues that interact with the N-cap did not reduce levels of fusion, except for substitutions made at residue I173. For N-cap and I173 mutants, the addition of chlorpromazine greatly promoted transfer of aqueous dye. Electrical capacitance measurements confirmed that fusion pores usually did not form for the I173 mutants. Thus, neither N-cap formation nor interactions with segment 173-178 are needed for hemifusion, but are required for reliable formation and enlargement of the fusion pore. It is proposed that binding of I173 into a deep hydrophobic cavity within the coiled-coil promotes the transition from hemifusion to fusion.