HIV is adept at evading attack by the immune system
Researchers have identified a cheap, commonly-used compound that, applied vaginally, can stop monkeys being infected with a primate version of HIV.
The discovery, by the University of Minnesota, raises hopes of a similar microbicidal treatment to block HIV transmission in humans.
Several microbicides have been tested, but results have been disappointing.
The study - focusing on a compound called glycerol monolaurate (GML) - is published online by the journal Nature.
GML is a naturally occurring compound widely used as an antimicrobial and anti-inflammatory agent in food and cosmetics.
Crucially, it is also cheap, and is likely to protect against other sexually transmitted infections too.
Lead researcher Dr Ashley Haase said that if GML proved to be effective in blocking HIV it could potentially help to save millions of lives.
A majority of cases of HIV worldwide are now contracted vaginally, and in Sub-Saharan Africa, where the pandemic is at its most intense, women account for nearly 60% of new infections.
After monkeys are exposed to their version of HIV, known as SIV, T-cells from the immune system rush to the scene to try to fight infection.
However, this is actually counter-productive, as the virus merely uses these cells as fuel to aid its expansion throughout the body.
Therefore, blocking this initial immune response - although it seems counter-intuitive - might actually help to stop infection in its tracks.
The Minnesota team examined GML because it has already been used to block a potentially lethal bacterial infection, and was known to impact on key immune system molecules.
They developed a gel containing GML which could be easily applied vaginally.
Ten monkeys - five treated with the gel, and five untreated - were injected with doses of SIV large enough to infect 50% of cells.
Four hours later, the monkeys were again treated with GML and then given a second dose.
The researchers monitored the animals for evidence of SIV infection for two weeks.
If there was no evidence of infection, the procedure was repeated for a second time.
Four of the five untreated monkeys contracted SIV, but the infection was not seen in any of the animals who were treated with the GML gel.
The researchers stressed more work was needed before human trials could take place.
But researcher Professor Pat Schlievert said: "GML is exceptionally inexpensive, is widely used in foods and cosmetics, and is easy to formulate in many ways for vaginal use.
"The compound has been demonstrated in vitro to inhibit the growth of nearly all sexually-transmitted disease microorganisms and other causes of vaginal infections, without affecting normal bacteria.
"Its use by women may significantly improve overall vaginal health."
Keith Alcorn, of the HIV information service NAM, said: "Dozens of compounds are currently being investigated in animal studies as potential microbicides to prevent HIV infection.
"What is interesting about glycerol monolaurate is that it's very cheap and it may protect against a broad spectrum of sexually transmitted infections, not just HIV.
"It also looks like the sort of compound that could be made available over the counter, without the need for a prescription, which can only help uptake of any potential microbicide."
Genevieve Edwards, of the HIV charity Terrence Higgins Trust, welcomed the research, but she warned that other precautions should still be taken.
"If we're able to produce an effective microbicide, it would be a significant step forward in the fight against HIV.
"In the short term, promoting condom use and good sex education are essential to reduce transmission."