Vaginal microbicides are designed to inhibit the sexual transmission of HIV-1 and other disease pathogens. Two Phase I clinical trials were conducted to evaluate the safety and tolerance of the gel, PRO 2000/5, in sexually abstinent volunteers. Both demonstrated that multiple application of the gel was safe and well tolerated in healthy, sexually abstinent women but no clear association between the use of the gel and epithelial disruption was indicated. Because genital ulceration and inflammation may increase the risk of HIV infection, additional clinical evaluation of PRO 2000/5 gel is warranted, especially for study participants with active sexual behavior.