To determine the safety and immunogenicity of 200 mcg MN rgp160 vaccine (Immuno-AG) versus placebo, administered on two immunization schedules to healthy volunteers. Per 06/15/94 amendment, to determine the safety and immunogenicity of 800 versus 200 mcg given as a fourth immunization at 9 or 11 months after the third injection (i.e., at month 17).