Participants in this study will be randomly assigned to receive vaccine or placebo injections in the groin area or the upper arm. All participants will have three baseline visits for blood tests and sigmoidoscopies to measure baseline immune functions. After these visits, participants will receive weekly injections for 4 weeks. Groin injections will be given subcutaneously (under the skin) and upper arm injections will be given intramuscularly (into the muscle). Participants will have follow-up visits 5 and 11 months after the last immunization. Participants will have blood draws and sigmoidoscopies and will receive HIV risk reduction counseling throughout the study. Total length of participation will be approximately 14 months. Participants may continue to contact the study for HIV testing and study-related concerns for 1 year after study participation.