We have constructed a recombinant baculovirus expressing the herpes simplex virus type 1 (HSV-1) glycoprotein I (gI). Sf9 cells infected with this recombinant virus synthesized gI-related polypeptides with apparent molecular sizes of 52 and 56 kDa. The recombinant gI appeared to be glycosylated, since it was susceptible to both tunicamycin and endoglycosidase H, and the expressed gI was transported to the surface of infected cells as judged by indirect immunofluorescence. Antibodies to the recombinant gI raised in mice neutralized HSV-1 infectivity. Finally, we show here for the first time that vaccination with gI can protect mice against HSV-1 challenge.