Women are randomized to receive rgp120/HIV-1SF2 vaccine or MF59 placebo. Patients receive the first immunization between 16 and 24 weeks gestation and monthly thereafter until delivery, for a maximum of five immunizations. Patients may continue to receive the immunization regimen to which they were originally assigned at 3, 6, 9, and 12 months postpartum. Maternal follow-up continues until 18 months postpartum; infants are followed until age 18 months.