The study is a randomized, open label, phase II clinical trial directed at evaluating the immunogenicity (as a primary end-point) and the safety (as a secondary end-point), of the recombinant HIV-1 Tat vaccine in HIV-1 infected adult subjects, anti-Tat antibody negative, HAART-treated with chronic suppressed HIV-1 infection, CD4+ T cell counts >= 200 cells/microliter, levels of plasma viremia < 50 copies/ml in the last 6 months prior to the screening and without a history of virologic rebound. The immunogenicity of 3 or 5 immunizations of the two different vaccine doses (7.5 and 30 micrograms) of the Tat vaccine has been evaluated.