Herpes simplex virus (HSV) type 1 glycoprotein H is essential for fusion of virus envelopes with cellular membranes and for the fusion of an infected cell membrane with an uninfected neighbour. Previous studies have pointed to a requirement for certain amino acid residues of the cytoplasmic tail of gH in these processes. Results from transient transfection experiments suggested that the serine-valine-proline (SVP) motif in the cytoplasmic tail may be important for gH-mediated fusion. HSV recombinants expressing gH molecules with mutations in the cytoplasmic tail were constructed and analysed in terms of their abilities to fuse cellular membranes and to function in virus entry. Viruses containing a deletion of the SVP motif, or in which the valine residue of this triplet was replaced by alanine, entered cells less efficiently than wild-type virus and were unable to induce syncytium formation on Vero cells.