Drugs applied three hours after infection could protect female monkeys from a type of HIV, US scientists said.
The findings, published in Science Translational Medicine, could lead to new ways to fight HIV, which is continuing to spread globally.
Experts say large clinical trials would be needed to test any new treatment, and condoms remain the best defence.
Vaginal gels containing HIV medicines have had mixed success in human clinical trials.
In the latest research, a US team took a different approach, testing a new HIV treatment in monkeys that has the potential to work after HIV exposure.
They found the gel protected five out of six monkeys from an animal-human laboratory strain of HIV when applied shortly before or three hours after infection.
The Centers for Disease Control and Prevention in Atlanta, Georgia, which led the study, say it is a "proof of concept" in an animal model.
Dr Charles Dobard, of the division of HIV/Aids prevention, told BBC News: "It's a promising after-sex vaginal gel to prevent HIV infection.
"Studies still need to be done to look at the window [of opportunity] - is it six, eight, 24 hours?"
Read the full article here:BBC News