Sera from 110 healthy pregnant women in the third trimester and from 47 controls matched for age, sex and socio-economic status were tested for the presence of antibodies to three antigenic preparations believed to contain mainly alpha, beta, and gamma polypeptides of herpes simplex virus (HSV) type I. Sixty-five of the 110 pregnant women and 22 of the 47 controls possessed antibodies against HSV immediate early antigen, or alpha polypeptides. Among them, three women possessed antibodies against immediate early antigen only. The pregnant group had significantly more women with antibody against all of the three antigenic preparations (60/110) than did the control group (18/47). We conclude that in some women only early genetically determined viral products were produced and that the pregnant state affected the genetic expression of latently infecting HSV.