Pregnant women are randomized to receive an initial injection of MN rgp120 vaccine or alum placebo between week 16 and week 24 of gestation, followed by monthly booster injections concluding at the end of pregnancy, for a total of five injections. Patients may have optional booster immunizations (vaccine or placebo) at 3, 6, 9, and 12 months after delivery. Mothers and infants are followed through 18 months after delivery. Per 06/94 addendum, patients will be contacted once or twice per year for at least 5 years to check on health status of patient and child. PER 12/21/94 ADDENDUM, post-partum immunizations are discontinued.