A gel of antibodies seems to protect female monkeys from sexual transmission of the AIDS virus. Researchers say the findings support the idea of using chemicals as an alternative to condoms in people.

Scientists say an effective HIV vaccine is still years away. In the meantime, they're looking for ways to prevent the spread of the virus, which has now infected 42 million people worldwide.

Half of all new cases of HIV infection are in women.

The chemicals, called microbicides, could help save the lives of women in developing countries whose partners may refuse to use condoms.

Microbicides could work like spermicides used for birth control, advocates say.

In the study, researchers in the United States and Britain made a gel using one of the antibodies humans naturally produce against HIV.

Since monkeys do not get AIDS, the researchers used a combination of HIV and a monkey version, called SIV.

Microbiology and immunology Prof. John Moore of Cornell University in New York and his colleagues applied the gel up to two hours before they put in the virus.

The antibody prevented nine out of 12 rhesus monkeys from getting infected. In comparison, 12 out of 13 control monkeys that were given a fake treatment became infected.

The antibody does not protect against all strains of HIV and is expensive. But it specifically targets HIV rather than harming healthy cells, and is a proof of the concept, Moore said.

"These observations support the concept that viral entry inhibitors can help prevent the sexual transmission of HIV to humans," the team wrote in Monday's online issue of the journal Nature Medicine.

Researchers are investigating more than 50 potential microbicides. Some target not only HIV, but a range of other sexually transmitted diseases, such as syphilis and gonorrhea.