A type-specific monoclonal antibody that efficiently neutralises HSV-1 immunoprecipitated a glycoprotein of slightly greater electrophoretic mobility than gB from HSV-1 infected cells. Pulse and pulse chase experiments indicate that this glycoprotein is distinct from HSV-1 glycoproteins gB, gC, gD, and gE. This was confirmed by the reactions of LP11 with a series of intertypic recombinants the results of which indicate that the LP11 target gene is located close to the HSV-1 thymidine kinase gene between map positions 0.28 and 0.31. In accordance with the presently agreed convention this glycoprotein should be designated gH-1, and it may correspond to the 110K glycoprotein described by S. D. Showalter, M. Zweig, and B. Hampar (1981), Infect Immune. 34, 684–692. Antibody LP11 inhibits plaque formation when added to cell monolayers after infection suggesting that gH-1 may play a role in cell-to-cell spread of infectious virus.