The experimental drug, ZMapp, cured the animals even when administered five days after infection, while they were displaying severe symptoms.
All 18 rhesus macaques made a complete recovery, in contrast to three other untreated monkeys that quickly fell seriously ill and died.
Two U.S. doctors given the drug after they were infected with Ebola while working in Liberia subsequently recovered. It is not known, however, whether they were saved by the drug or just lucky.
About 45% of those infected in the current outbreak have survived without treatment.
At least two other patients treated with ZMapp have died, possibly because help got to them too late.
The new research, published in a special report on Nature Journal’s website, provides hard evidence that the drug works and can be highly effective.
ZMapp is a blend of three laboratory-made antibodies designed to neutralise the virus.