The only clinical trial data on the experimental Ebola drug ZMapp shows it is 100 percent effective in monkey studies, even in later stages of the infection, the journal Nature reports.
The researchers, publishing their data in Nature, said it was a "very important step forward", the BBC reported.
Yet the limited supplies will not help the 20,000 people predicted to be infected during the outbreak in West Africa.
And two out of seven people given the drug, have later died from the disease.
ZMapp has been dubbed the "secret serum" as it is still in the experimental stages of drug development with, until now, no public data on effectiveness.
Doctors have turned to it as there is no cure for Ebola, which has killed more than 1,500 people since it started in Guinea.
Researchers have been investigating different combinations of antibodies, a part of the immune system which binds to viruses, as a therapy.
Previous combinations have shown some effectiveness in animal studies. ZMapp is the latest cocktail and contains three antibodies.
Trials on 18 rhesus macaques infected with Ebola showed 100 percent survival.
This included animals given the drug up to five days after infection. For the monkeys this would be a relatively late stage in the infection, around three days before it becomes fatal.
Meanwhile. the West African state of Senegal is the fifth country to be hit by the Ebola outbreak.
The World Health Organisation (WHO) says Ebola cases rose last week at the fastest pace since the epidemic began Africa in March.
The epidemic has defied efforts by governments to control it, prompting the leading charity fighting the outbreak, Medecins Sans Frontieres, to call for the U.N. Security Council to take charge of efforts to stop it.