A drug used to treat patients with multiple sclerosis may also be effective against the deadly Sars virus, according to scientists.
There is still no effective cure for Sars
Researchers in Germany say beta interferon could be "the drug of choice" in the fight against Sars.
Laboratory tests have shown the drug can help to stop the virus from spreading and may also protect patients from catching it again.
However, the UK's Health Protection Agency played down the findings saying much more research is needed.
No effective treatment
There is currently no effective treatment or vaccine for Sars.
The World Health Organization recommends that patients with suspected Sars are treated with anti-pneumonia drugs.
Many hospitals used an anti-viral called Ribavirin to treat patients during the recent outbreak, which killed over 800 people and infected over 8,000 around the world.
However, it was not completely effective and the WHO has called for a large-scale study to see if it or other drugs are more effective at treating the virus.
Professor Jindrich Cinatl and colleagues from Frankfurt University Medical School tested a group of drugs known as recombinant interferons on two strains of Sars.
Samples were obtained from patients who were admitted to hospital with the virus in Hong Kong and from others who were admitted the Frankfurt University clinical centre earlier this year.
They found that beta interferon was effective against both strains in test tubes.
Other types of these drugs, namely alpha interferon and gamma interferon, were less effective.
The scientists said their findings could help doctors trying to treat patients with the disease.
Experts have warned that Sars is likely to re-appear later this year with the arrival of the traditional flu season in the autumn.
Writing in the medical journal The Lancet, the scientists said: "Interferon beta could be the drug of choice, alone or in combination with other antiviral drugs, in the treatment of Sars."
But Professor Robert Read, a member of Health Protection Agency's expert advisory group on Sars, said further research is needed.
"This is an interesting piece of research but it is limited," he told BBC News Online.
"Just because a drug kills the virus in a test tube does not mean it will kill the virus in patients.
"It may well be that beta interferon will be a useful drug against Sars but until we are able to use it in patients it is impossible to be as descriptive as the authors of this paper suggest."
He added that other antivirals including anti-Aids drugs had also shown promise in laboratory tests.
But he said these must also be tested on patients before they can be regarded as an effective treatment against Sars.