A monoclonal antibody, B1C1, binding to an epitope of antigenic site II of the herpes simplex virus type 1 (HSV-1) glycoprotein gC-1, is a potent inhibitor of two important biological functions of gC-1: its binding to cell surface heparan sulfate and its binding to the receptor for complement factor C3b. Here, we have analyzed a B1C1-resistant HSV-1 variant (HSV-12762/B1C1B4.2), obtained after passage of wild type HSV-1 (HSV-12762) in the presence of high concentrations of B1C1. The transport of newly synthesized mutant gC-1 to the cell surface was comparable to that of wild type glycoprotein, but no binding of surface-associated mutant gC-1 to B1C1 was detected. However, mutant and wild type gC-1 bound equally well to other site II Mabs. Attachment of wild type but not mutant virus was inhibited by B1C1. Sequencing of the mutant gC-1 gene revealed only one nucleotide change, resulting in replacement of Thr150 by an Ile, in turn destroying an N-glycosylation site at Asn148. Loss of one complex type N-linked glycan was confirmed by endoglycosidase digestion and subsequent SDS-polyacrylamide gel electrophoresis. Circular dichroism analysis of purified gC-1 from cells infected with mutant or wild type virus did not reveal any difference in secondary structure between mutant and wild type gC-1. It was not possible to obtain a B1C1-resistant phenotype by nucleotide-directed mutagenesis of gC-1 where Asn148 was changed to a glutamine. These data demonstrated that the threonine of the glycosylation site and not the N-linked glycan in itself was essential for B1C1 binding