The contributions of a set of herpes simplex virus type 1 membrane proteins towards the process of cell-cell fusion were examined with a series of deletion mutants into which a syncytial mutation had been introduced at codon 855 of the glycoprotein B (gB) gene. Analysis of the fusion phenotypes of these recombinant viruses in Vero cells revealed that while gC, gG, US5, and UL43 are dispensable for syncytium formation at both high and low multiplicities of infection, gD, gHgL, gE, gI, and gM were all required for the fusion of cellular membranes. These data confirm that the requirements for virion entry and cell-cell fusion are not identical. gD and gHgL, like gB, are essential for both processes. gG, gI, and gM, on the other hand, are dispensable for virus penetration, yet play a role in cell-to-cell spread by the direct contact route, at least on an SC16 gBANG background.