Participants will have 11 study visits over 18 months; the total duration of the study will be 30 months. The length of visits will vary and may last up to 3 hours. Study visits will include a medical interview, brief physical exam, and blood and urine tests. Participants will be tested for HIV before entering the study and at least five times during the study. All vaccine and placebo injections will be given in the upper arm muscle.