ORIGINAL DESIGN: Fifty-seven adult subjects will be randomized to receive MN rgp120 vaccine at one of three dosages (100, 300, or 600 mcg), or 300 mcg MN rgp120 vaccine given concurrently with 300 mcg IIIB rgp120 vaccine, or placebo. Twelve subjects will be entered onto each of the four vaccine arms, and nine subjects will be entered on the placebo arm. Immunizations (or placebo injections) are given intramuscularly at 0, 4, 24, and 48 weeks. Subjects are followed for 15 months after the first immunization.