In a statement, People for the Ethical Treatment of Animals (Peta) said it had been told by United Airlines that it would ban the transport of primates to laboratories under the terms of its cargo policy.
"The new policy reads, 'We do not book, accept or transport primates to or from medical research facilities'," it said.
United Airlines, responding to an email from AFP, confirmed that it would no longer transport what it called "non-human primates" to or from medical labs in the United States or abroad.
It also said it did not ship primates between zoos and other sanctuaries within the United States.
With Air Canada in the process of implementing a similar ban, Peta said "not a single major airline based in North America" will now deliver monkeys to labs where they are liable to be used in experiments.
"Peta will continue to pressure the few overseas airlines - now numbering only four - that continue this inhumane practice," said Peta senior vice president Kathy Guillermo.
Peta identified the four as Air France, China Eastern Airlines, Philippine Airlines and Vietnam Airlines.
United previously banned the shipping of primates, but came under pressure from Peta supporters when it merged in 2010 with Continental Airlines, which did allow for such shipments.
Getting airlines to cease transporting primates is part of Peta's broader campaign against the use of animals for lab tests.