Scientists have discovered a protein in monkeys that can block infection by the virus that causes Aids.
The team, from the US Dana-Farber Cancer Institute, hope their work could lead to new ways to prevent humans being infected with HIV.
They believe a similar molecule known to exist in humans might have the potential to stop HIV in its tracks.
However, writing in Nature, they say a way has to be found to make the human form as potent as the monkey version.
The molecule, named TRIM5-alpha appears to patrol the body for viruses, and, if they enter a cell, prevents them from causing harm.
Dr Anthony Fauci, of the US National Institute of Allergy and Infectious Diseases, which funded the study, said: "Identification of this HIV-blocking factor opens new avenues for intervening in the early stage of HIV infection, before the virus can gain a toehold."
Human cells contain a similar TRIM5-alpha protein - but it is less effective than the monkey version in blocking HIV infection.
It is possible that the potency of TRIM5-alpha differs among individuals because of genetic variations.
This might explain why some people infected with HIV progress rapidly to Aids, while others have remained healthy for decades.
The key to exploiting the molecule in new drugs will be to find ways to increase its effectiveness at blocking HIV.
Alternatively, scientists have not ruled out giving patients the more potent monkey version.
Human HIV cannot infect baboons
For HIV to reproduce itself once it has infected a cell it must first shed its protective coating - called a capsid - to release its genetic material.
The researchers have shown that TRIM5-alpha works by preventing the protective coating from being jettisoned.
This renders the virus unable to hijack the genetic machinery in the host cell - and thus unable to trigger the production of new copies of itself.
Professor Frances Gotch, an expert in HIV at Imperial College London, told BBC News Online, the finding was "very exciting".
"It is a basic discovery, a new mechanism that we did not know about before," she said.
"People have been wondering for a long time how the virus uncoats itself in the cell.
"This is not a cure for Aids, but I could imagine it leading to a prophylactic treatment which people at high risk might consider using in the same way as people take anti-malarials when they go to an area where malaria is a problem."
Professor Gotch said it was possible that some people carried a form of the human molecule with a genetic abnormality which made it more potent, and thus gave them greater protection from HIV infection.
Keith Alcorn, editor, of the website aidsmap.com, said: "There are two possible avenues for developing a treatment based on this discovery.
"One is to boost the human body's own production of this protein, the other is to make a copy that can be given as a medicine."
Lisa Power, head of policy at the HIV charity the Terrence Higgins Trust, said: "This is a promising lead, but it's at a very early stage - it will be some time before they know whether or not this will work in humans.
"But there is still no vaccine or cure for HIV, so research like this should be encouraged."