Topical microbicides, substances that kill microbes, are being studied to prevent sexual transmission of HIV and other disease-causing agents. In the future, topical microbicides may be applied vaginally to prevent both acquisition and transmission of HIV and other sexually transmitted infections (STIs). The purpose of this study is to assess whether there is a measurable response to daily applications of a topical microbicide, 1% tenofovir gel, in women at low risk for HIV infection.