 During the 20 months of the study, volunteers will be asked to donate blood up to 14 times, undergo seven flexible sigmoidoscopies with biopsies, and receive three pairs of vaccine injections. For up to 36 days after each experimental vaccination, volunteers will be asked to abstain from activity that might spread the virus in the vaccine and put others at risk. Volunteers will receive specific instructions on how to avoid these risks. Volunteers will also be asked not to engage in receptive anal intercourse during the study. 