A vaginal gel has been developed in the US which can block the Aids virus, scientists have said.
The microbicide medicine, trialled in a study in South Africa, is said to halve the chances of a woman catching HIV from an infected man.
More research is needed to confirm the initial results, although the amount of protection afforded by the gel may not be enough to guarantee approval in countries such as the US, the researchers said.
Michel Sidibe, from the World Health Organisation's UNAids programme, said the research is "giving hope to women", who make up the most new HIV infections, and that a gel could "help us break the trajectory of the Aids epidemic".
Dr Anthony Fauci, from the US National Institutes of Health, said this is "the first time we've ever seen any microbicide give a positive result".
The gel contains the Aids drug tenofovir and was seen to reduce HIV infection risk after a year of use by as much as 50% and after two-and-a-half years by 39%, compared with a gel containing no medicine.
The chances of catching HSV-2, the virus which leads to herpes of the genital areas, was also said to have been cut by half with the new gel. The researchers said this is another breakthrough because the risk of catching HIV is increased with sexually transmitted diseases.
The US journal Science published the results of the study online.
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