Volunteers are randomized to receive 200 mcg MN rgp160 or placebo at months 0, 1, and 6 or at months 0, 2, and 8. For each immunization schedule, ten volunteers receive vaccine and two volunteers receive placebo. Per amendment, volunteers receive a fourth immunization of 800 or 200 mcg (or placebo) at 9 or 11 months after the third injection (i.e., at month 17) and are followed for 6 months afterward.