A drug manufactured from the saliva of vampire bats could make it easier to protect the brain from stroke damage.
Normally, most stroke patients should get into hospital for "clot-busting" treatment within a few hours.
However, it appears the drug Desmoteplase can make a difference even if given nine hours after the stroke.
At an international stroke conference in San Diego, US, researchers claimed it was the "biggest breakthrough" in stroke treatment in 20 years.
Most strokes happen when a blood clot lodges and blocks a blood vessel within the brain.
This deprives brain cells in that area of the oxygen they need to survive, often leading to brain damage which causes paralysis or weakness in various parts of the body.
Doctors have long stressed the importance of getting this kind of stroke victim treated as soon as possible after the stroke, as it has been shown that long-lasting damage can be reduced if drugs are given to break up the clot and restore blood flow quickly.
However, in the UK, the majority of stroke patients do not receive this thrombolysis treatment within the "window" of a few hours after the stroke.
The latest study, carried out at 44 hospitals in Europe, Australia and Asia, looked at more than 100 patients who were either given standard treatment or a new drug, Desmoteplase.
It is based on a protein extracted from the saliva of vampire bats, who use it to stop their victim's blood clotting while they are feeding.
Crucially, Desmoteplase appears to be able to break up the clots in the brain without adversely affecting blood clotting in other parts of the body - and most importantly, without increasing the risk of "bleeding on the brain".
The patients in the trial given the vampire bat drug benefited even when they only received it nine hours after the stroke.
The patients were assessed prior to being given the drug using an MRI scanner, and those most likely to benefit picked out.
This could create problems of access in NHS hospitals - but the widening of the treatment "window" to nine hours would in theory make it much easier for patients to get treated in time.
This could mean that they end up with fewer long-term disabilities from their stroke.
Dr Howard Rowley, from the University of Wisconsin Medical School, said: "The results of the study are exciting because they show specifically how the use of the drug significantly extends the time during which treatment can begin after the onset of stroke symptoms, from three hours to nine hours.
"The impact of these results cannot be overstated - buying more time to treat stroke symptoms means we can give acute stroke victims hope for a better outcome.
"This is the biggest breakthrough I've seen in 20 years."
It is hoped that the drug will gain Food and Drug Administration clearance in the US after results of a further, larger trial due to start within the next few months.