Participants will have about 10 clinic visits during the study. Most visits will last about 2 hours; those on vaccination days will last about 4 hours. At each visit, participants will be checked for health changes or problems and will be asked about medications they are taking. Blood will be drawn for immune system testing. Additional laboratory tests may be requested between visits. Participants will be tested several times for HIV, will be questioned about their sexual behavior and drug use, and about social effects they may have experienced from their participation in the study.