Healthy, adult volunteers without identifiable high-risk behavior for HIV-1 infection are vaccinated. In phase 1, at each participating unit, four volunteers receive a dose of gp160 (12.5 mcg); two volunteers receive placebo. Volunteers are monitored 1 month before proceeding to the second phase. In phase 2, four volunteers receive gp160 (50 mcg); two volunteers receive placebo. Primary immunization and two booster immunizations at day 30 and day 180 are done in an outpatient setting. Volunteers are closely monitored for the first 2 weeks postimmunization (primary and boosters), and extensively followed for a minimum of 2 years. Volunteers may be offered an additional boost of the same preparation at 18 months. Per 07/28/92 amendment, 18 volunteers will receive 200 mcg gp160 in an unblinded study. Volunteers in phase 2 who received four doses of 50 mcg gp160 receive an additional boost of 200 mcg gp160 at 18-19 months post initial vaccination. Per 05/13/94 amendment, volunteers in phase 2 who received the additional 200 mcg boost receive another 200 mcg boost 18-24 months after the last injection (St. Louis University site only).