We used a murine monoclonal antibody to herpes simplex virus (HSV) type 2 (HSV-2) glycoprotein G (gG) to develop an enzyme immunoassay that detected HSV-2 type-specific antibodies in human sera. Antibodies to HSV-2 gG were detected in 98 (96%) of 102 sera from pregnant women with culture-proved HSV-2 infection. Sixty-five percent of the women had serological evidence of past HSV-2 infection by the Rawls index, based on titers of neutralizing antibody to HSV type 1 and HSV-2. Thirty (88%) of 34 infants exposed to maternal HSV infection at delivery had antibodies to HSV-2 gG and remained well. One infant exposed to primary maternal HSV-2 infection lacked antibodies to HSV-2 gG and developed neonatal HSV-2 infection. The mean +/- SD optical density by HSV-2 gG enzyme-linked immunosorbent assay for sera obtained from 17 infants within one week after onset of neonatal HSV-2 infection was 0.25 +/- 0.12, compared with 1.15 +/- 0.34 in cord blood sera from exposed infants who did not develop symptoms (P less than .0001 by t test).