A vaccine for the prevention of HIV infection remains an urgent need as part of the efforts to control the HIV pandemic. In this phase III efficacy trial, a 'prime-boost' vaccine strategy is evaluated for prevention of infection and ameliorationn of disease course. ALVAC-HIV (vCP1521) from sanofi pasteur is given as the 'prime' vaccine at months 0, 1, 3 and 6; AIDSVAX gp120 B/E from VaxGen is given as the 'boost' at months 3 and 6. This regimen will be given to 8,000 adult Thai subjects, while another 8,000 will be given placebos in a double-blinded, randomized manner. Following the completion of each subjects immunization phase, he/she will be followed for 3 years with clinic visits every 6 months with HIV testing, pre- and post-test counseling. Subjects who become HIV infected will be counseled, referred to HIV treatment facilities for management according to national guidelines, and offered enrollment in a protocol for extended follow-up.