Four monoclonal antibodies to glycoprotein D (gD) of herpes simplex virus (HSV) types 1 and 2 neutralized virus in the presence of complement but exhibited diverse activities in its absence. Amino acid substitutions that conferred resistance to neutralization by each antibody were identified by deriving the nucleotide sequence of the gD gene from resistant mutants. Each antibody selected a substitution from different parts of the molecule and mutants resistant to a single antibody always arose from the same mutation. One of the antibodies reacted with a synthetic oligopeptide corresponding to the region of the molecule in which amino acid substitution conferred resistance, but the remaining three antibodies failed to react with predicted oligopeptide targets. These antibodies may therefore react with 'discontinuous' epitopes, a view supported by the observation that two of these three antibodies competed with each other in binding assays despite the fact that substitutions conferring resistance to neutralization arose nearly 100 residues apart in the primary sequence. The four antibodies had very different biological properties. One antibody neutralized infectivity but did not inhibit cell fusion, one antibody inhibited cell fusion but did not neutralize, while a third antibody had both activities. One antibody had neither activity but enhanced the infectivity of HSV-2 in a type-specific manner. The ability of antibodies to inhibit cell fusion by syncytial virus strains correlated with an ability to prevent plaque enlargement by a non-syncytial virus strain, implying a role for gD in the intercellular spread of virus that is independent of the syncytial phenotype. We found no correlation between neutralizing activity and anti-fusion activity suggesting that, while gD is involved in cell fusion, it has at least one other function which is required for infectivity.