"The level of improvement was utterly beyond my honest expectation," said one study leader, Gary Kobinger of the Public Health Agency of Canada in Winnipeg.
"For animal data, it's extremely impressive," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which had a role in the work.
It's not known how well the drug would work in people, who can take up to 21 days to show symptoms and are not infected the way these monkeys were in a lab.
Several experts said it's not possible to estimate a window of opportunity for treating people, but that it was encouraging that the animals recovered when treated even after advanced disease developed.
The study was published online Friday by the journal Nature.
ZMapp had never been tested in humans before two Americans aid workers who got Ebola while working in Africa were allowed to try it. The rest of the limited supply was given to five others.
There is no more ZMapp now, and once a new batch is ready, it still needs some basic tests before it can be tried again during the African outbreak, Fauci said. "We do need to know what the proper dose is" in people and that it's safe, he said.
Ebola has killed more than 1,500 people this year and the World Health Organization says there could be as many as 20,000 cases before the outbreak is brought under control. On Friday, it spread to a fifth African country—Senegal, where a university student who traveled there from Guinea was being treated.