The majority of HIV infected people worldwide became infected with the virus through heterosexual contact. BufferGel and PRO 2000 Gel are vaginal gels designed to prevent women from becoming infected with HIV during sexual intercourse with an HIV infected partner. This study will test the safety and effectiveness of these gels.

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).

BufferGel is a buffering agent designed to maintain normal vaginal acidity in the presence of ejaculate. Studies have shown that HIV is inactivated below a pH of 4 to 5.8. Carbopol 974, the major nonaqueous component of BufferGel, is commonly used as a gelling or tableting agent and has a well-documented safety record. BufferGel has proven safe and well tolerated in Phase I studies. PRO 2000/5 inhibits viral entry into susceptible cells. PRO 2000/5 has also been evaluated in Phase I studies and was found to be safe and well tolerated.

Participants in this study will be randomly assigned to receive one of four interventions: BufferGel, PRO 2000/5, placebo gel, or no gel. All participants will receive HIV risk reduction counseling, condoms, and diagnosis and treatment of sexually transmitted diseases. Participants in the three gel arms will be given single-use, prefilled applicators of gel and will be instructed to apply one dose of the product (BufferGel, PRO 2000/5, or placebo gel) intravaginally up to 60 minutes before each vaginal intercourse while in the study.

Participants will be enrolled for approximately 12 to 30 months, depending on when they enter the study. Participants will have two screening study visits and then monthly visits. Monthly study visits may take place at a clinic, the participant's home, or other community site. Monthly visits will include a medical interview; urine testing, including pregnancy testing; HIV and sexually transmitted infection risk reduction counseling; and distribution of study supplies (condoms and gel-filled applicators). Blood collection will occur at selected monthly visits. Four times a year, participants will be seen in the clinic for a more structured medical interview, pelvic exam, and HIV testing. At one of these visits, participants at particular sites who are willing will be asked to complete a behavioral risk assessment. At some visits vaginal fluid swabs will occur. Participants at non-US study sites will be asked for permission for these swabs to be stored for further testing. Assessment interviews will be both face to face and self-reporting.

Note: Per the 12/15/06 amendment, all study participants will now enter the Phase IIb study.

Inclusion Criteria:

• HIV uninfected
• Have had sexual intercourse at least once in the 3 months prior to study entry
• Able to provide adequate contact information to study officials for purposes of follow-up

Exclusion Criteria:

• History of adverse reaction to latex
• Nonmedical injection drug use in the 12 months prior to study entry
• Vaginal intercourse more than an average of two times per day in the 2 weeks prior to study entry
• Plan to become pregnant in the 30 months after study entry
• Plan to travel away from the study site for more than 3 consecutive months in the 30 months after study entry
• Plan to relocate away from the study site in the 30 months after study entry
• Participation in another clinical trial of a vaginal product
• Pregnant within 42 days of study entry
• Have a sexually transmitted disease or other reproductive tract infection diagnosed by study staff
• Abnormal pelvic exam indicating deep epithelial disruption
• Condition that, in the opinion of the investigator, may interfere with the study
• Liver or kidney function abnormality of Grade 3 or higher
• Blood or blood clotting abnormality of Grade 4 or higher