Participants will be randomly assigned to one of three groups and will be followed for one year. Study injections will be given by needle-free intramuscular injection at the start of study and at Months 1 and 2. Group 1 will receive 3 injections of the study vaccine; Group 2 will receive 2 injections of the study vaccine (at start and Month 2) and injection of placebo (at Month 1); Group 3 will receive 3 injections of placebo. After a screening visit, study visits will occur at enrollment (initial injection) followed by 5 visits every 14 days for the first 2.5 months, with three additional visits at Months 6, 9, and 12. All participants will undergo physical exams, blood and urine tests to assess measures of health, and blood tests to assess HIV infection and immune response to the injections.