Clinical trials of a new drug designed to help prevent women contracting the Aids virus have been stopped.
African women are often unable to make men use condoms
The World Health Organization said the drug, which uses a microbicidal gel, did not help the women and made them more vulnerable.
The tests were carried out on more than 1,300 women in South Africa, Benin, Uganda and India.
The WHO and the United Nations Aids agency said it was not clear why the product did not work.
A similar test in Nigeria has also been halted but trials of three other microbicides are still continuing.
The cellulose sulphate gel, based on cotton and made by Canadian company Polydex Pharmaceuticals, was supposed to release an active ingredient designed to kill HIV during sexual intercourse.
"This is a disappointing and unexpected setback in the search for a safe and effective microbicide that can be used by women to protect themselves against HIV infection," said WHO and UNAids in a joint statement.
Around 30 women had contracted HIV since the trial started in 2005, the AFP news agency quotes study co-ordinator Tim Farley as saying.
Scientists had hoped that microbicides could have a major impact in the fight against Aids, especially in Africa, where women bear the brunt of the disease.
They are being developed because it is often difficult for women to insist that men use condoms during sex.
"The closure of these trials is a stark reminder that drug development in general is a difficult and unpredictable process, and we must constantly bear in mind that the majority of drugs that enter the clinical trial process fail," said Zeda Rosenberg, the head of the International Partnership for Microbicides (IPM).
The International Aids Society said it was extremely disappointed at the setback, but that lessons would be learnt.
"This will strengthen future microbicide research and increase our overall knowledge of how such compounds work," IAS President Pedro Cahn told AFP news agency.