This is an event driven study that plans to randomize seronegative women. Participants will be randomized to a 1:1 ratio to receive 1% tenofovir gel or placebo gel. Each will be asked to insert a dose of the assigned study product within 12 hours prior to a coital event and another dose as soon as possible within 12 hours after a coital event. Participants will be advised to use only two doses of gel in a 24 hour period.