The genome of herpes simplex virus 1 consists of two components, L and S, each containing unique sequences flanked by inverted repeats. Current and earlier studies have shown that 11 of the 12 open reading frames contained in the unique sequences of the S component can be deleted and are dispensable for growth in cell culture. Analyses of one recombinant virus containing a deletion in the open reading frame US7 permitted the identification of a monoclonal antibody specific for the product of this gene. The protein encoded by this gene has a predicted translated molecular weight of 41,366 and an apparent molecular weight of approximately 65,000 in denaturing polyacrylamide gels. The electrophoretic mobility of the protein synthesized by cells in the presence of inhibitory concentrations of tunicamycin is faster than that of the protein accumulating in lysates of untreated infected cells. We conclude that the product of US7 is a glycoprotein subject to N-linked glycosylation, and we have designated it glycoprotein I. These studies indicate that the unique sequences of the S component encode four glycoproteins (G, D, I, and E) of which at least three (G, I, and E) are dispensable for growth in continuous lines of primate cells.