Recent Phase I trials conducted at the AIDS Vaccine Units have shown that antibodies have persisted in most recipients for 6 months after boosting, and responses seem significantly higher and more persistent than responses achieved by just two doses of soluble protein vaccine alone or two doses of HIVAC-1e alone. This study tests in a previously recruited cohort of volunteers whether additional boosting with soluble recombinant gp160 results in increased immunogenicity of longer duration.