Gene US4 of herpes simplex virus type 1 (HSV-1) has been predicted, from DNA sequence analysis, to encode a protein of molecular weight 25237 and its properties suggest it to be a membrane-associated protein. We have investigated this protein by raising antiserum to a synthetic oligopeptide corresponding to a stretch of amino acids from an internal hydrophilic region of the predicted sequence. This antiserum immunoprecipitates three glycoprotein species of apparent mol. wt. 37 000, 48 000 and 56 000 from extracts of cells infected with HSV-1. These species are also specifically immunoprecipitated from purified virions. The in vitro translation product of gene US4 has an apparent mol. wt. of 35 000. Sequence comparisons of the short unique regions of the HSV-1 and HSV-2 genomes, in combination with published mapping data for glycoprotein G (gG) of HSV-2, has led to the conclusion that the product of gene US4 of HSV-1 is the equivalent of gG.