Results demonstrating involvement of glycoprotein C (gC) of herpes simplex type 1 virus (HSV-1) in attachment of the virus to the cell are presented. Monoclonal antibodies against gC-1 inhibited adsorption of gC(+)-strains. The gC(-)-mutant, MP, attached to cells but at a reduced rate. Attachment of the MP-mutant was unaffected by presence of anti-gC-1 antibody. Purified truncated gC-1 adsorbed to cells at a rate essentially the same as that of gC(+)-virus. Glycoprotein C-1 pretreated with heparin did not adsorb to cells. The results are compatible with a suggested role for gC in HSV attachment.