Accessible, usable, and effective vaginal microbicides could significantly decrease the spread of HIV. Vaginal microbicides could be used with condoms to prevent HIV infection; they could also be used in situations in which an individual is unable to negotiate condom use. Vaginal microbicides are potentially bidirectional, preventing both male-to-female and female-to-male HIV transmission. This study will evaluate the safety and effectiveness of two vaginal microbicides, BufferGel and PRO 2000/5, in preventing the transmission of HIV. The study will also evaluate the effectiveness of these gels in preventing other common sexually transmitted infections (STIs).