Ebola viruses (EboV) are enveloped RNA viruses infecting cells by a pH-dependent process mediated by viral glycoproteins (GP) involving endocytosis of virions and their routing into acidic endosomes. As with wellcharacterized pH-dependent viral entry proteins, in particular influenza virus hemagglutinin, it is thought that EboV GP require activation by low pH in order to mediate fusion of the viral envelope with the membrane of endosomes. However, it has not yet been possible to confirm the direct role of EboV GP in membrane fusion and the requirement for low-pH activation. It was in particular not possible to induce formation of syncytia by exposing cells expressing EboV GP to acidic medium. Here, we have used an assay based on the induction of a -galactosidase (lacZ) reporter gene in target cells to detect cytoplasmic exchanges, indicating membrane fusion, with cells expressing EboV GP (Zaire species). Acidic activation of GP-expressing cells was required for efficient fusion with target cells. The direct role of EboV GP in this process is indicated by its inhibition by anti-GP antibodies and by the lack of activity of mutant GP normally expressed at the cell surface but defective for virus entry. Fusion was not observed when target cells underwent acidic treatment, for example, when they were placed in coculture with GP-expressing cells before the activation step. This unexpected feature, possibly related to the nature of the EboV receptor, could explain the impossibility of inducing formation of syncytia among GP-expressing cells.Filoviruses, comprising the Marburg virus and Ebola virus (EboV), the latter subdivided into four species, are negativestranded enveloped RNA viruses responsible for lethal hemorrhagic disease in humans and nonhuman primates (reviewed in references 13, 19, and 33). The nonsegmented genome comprises seven genes, with gene 4 encoding glycoproteins (GP) apparently responsible for the cell entry process. It is indeed possible to infect a wide variety of mammalian cells, with the notable exception of lymphocytes and derived cell lines, with GP-coated pseudotypes derived from vesicular stomatitis virus (VSV) or human immunodeficiency virus type 1 (HIV-1) (31, 39). The functional form of filovirus GP appears to be a complex of two subunits, GP1 and GP2, which are the amino-and carboxy-terminal cleavage products of an ϳ680-amino-acid precursor (14, 35). In the case of EboV, the precursor GP is translated from a subset of viral mRNAs edited in a polyuridine sequence to produce a Ϫ1 frameshift and is therefore less abundant than an ϳ360-amino-acid soluble glycoprotein of unknown function translated from unedited mRNAs (14). However, certain EboV strains from the Zaire species with an additional uridine in the frameshit region express predominantly the full-length GP (34).The overall organization and processing of filovirus GP indicate their homology with other viral proteins mediating cell entry, such as the influenza virus hemagglutinin (HA) processed into HA1 and HA2 subunits and the precu...