Monoclonal antibodies HCl and HD1, directed against herpes simplex virus type 1 (HSV-1) glycoproteins gC and gD, respectively, were evaluated for their ability to passively immunize mice against acute virus-induced neurological disease after footpad inoculation with HSV-1 or herpes simplex virus type 2 (HSV-2). Control virus-infected mice receiving a single intraperitoneal injection of normal serum died within 7 to 10 days after the spread of virus from footpad to spinal cord and brain. However, a single intraperitoneal injection of either HCl or HD1 antibody protected mice from neurological illness and death when administered to HSV-1 (strain HTZ)-infected mice at either 2 h before virus challenge or at 24 h after virus inoculation. To determine the in vivo specificity of the antibodies, passive transfer studies were performed with mice infected with the MP strain of HSV-1, a mutant of HSV-1 (mP) which is defective in the production of glycoprotein gC. Whereas HD1 antibody decreased the incidence of neurological illness in MP- and mP-infected mice, HCl antibody, which protected mP-infected animals, failed to protect mice infected with the MP strain. When HD1 antibody was administered to HSV-2 (strain G)-infected mice at either 2 h before virus challenge or at 6 h (but not 24 h) after virus inoculation, 100% of the infected animals receiving HD1 antibody survived. In contrast, 100% of HSV-2 (strain G)-infected animals passively immunized with HCl antibody developed neurological illness and died. These results provide in vivo evidence that the HSV-induced glycoprotein gC expresses type-specific antigenic determinants, whereas glycoprotein gD expresses type-common determinants.