Of the patients given the two more effective drugs, 29 percent on REGN-EB3 and 34 percent on mAb114 died, NIAID said.
In contrast, 49 percent on ZMapp and 53 percent on Remdesivir died in the study, the agency said.
The survival rate among patients with low levels of the virus in their blood was as high as 94 percent when they were given REGN-EB3, and 89 percent when on mAb114, the agency said.
The World Health Organisation (WHO) co-ordinated the trial, which began in November last year.
Hailing the success of the study, Jeremy Farrar, director of the Wellcome Trust global health charity, said the treatments would "undoubtedly save lives".
The findings, Mr Farrar said, indicate scientists are getting closer to turning Ebola into a "preventable and treatable" disease.
"We won't ever get rid of Ebola but we should be able to stop these outbreaks from turning into major national and regional epidemics," he added.
The current outbreak in eastern DR Congo began in August last year and is the biggest of the 10 to hit the country since 1976, when the virus was first discovered.
But it is dwarfed by the West African epidemic of 2014-16, which affected 28,616 people mainly in Guinea, Liberia and Sierra Leone. About 11,310 people died in what was the largest outbreak of the virus ever recorded.