WASHINGTON – It’s official: The National Institutes of Health plans to end most use of chimpanzees in government medical research, saying humans’ closest relatives “deserve special respect.”

The NIH announced Wednesday that it will retire about 310 government-owned chimpanzees from research over the next few years, and keep only 50 others essentially on retainer — available if needed for crucial medical studies that could be performed no other way.

“These amazing animals have taught us a great deal already,” said NIH Director Dr. Francis Collins. He said the decision helps usher in “a compassionate era.”

The NIH’s decision was long expected, after the prestigious Institute of Medicine declared in 2011 that nearly all use of chimps for invasive medical research no longer can be justified. Much of the rest of the world already had ended such research.

Any future biomedical research funded by the NIH with chimps, government-owned or not, would be allowed only under strict conditions after review by a special advisory board. In five years, the NIH will reassess if even that group of 50 still is needed for science.

“This is an historic moment and major turning point for chimpanzees in laboratories, some who have been languishing in concrete housing for over 50 years,” said Wayne Pacelle, president and CEO of The Humane Society of the United States. “It is crucial now to ensure that the release of hundreds of chimpanzees to sanctuary becomes a reality.”

What’s unclear is exactly where the retiring chimps, which have spent their lives in research facilities around the country, now will spend their final years. NIH said they could eventually join more than 150 other chimps already in the national sanctuary system operated by Chimp Haven in Louisiana.

But NIH officials said currently there’s not enough space to handle all of the 310 destined for retirement, so they’re exploring additional locations.

The other hurdle is money: Congress limited how much the NIH can spend on caring for chimps in the sanctuary system. Negotiations are under way to shift money the agency has spent housing the animals in research facilities toward supporting their retirement.

One chimp center, the Texas Biomedical Research Institute, said keeping just 50 of the animals for ongoing research isn’t enough and could hamper efforts to fight not just human illnesses but diseases that kill apes, too.

Moreover, moving retired chimpanzees to the federal sanctuary “would take them away from their caregivers, many of whom they have known all of their lives,” said an institute statement that argued the animals would fare better if they stayed put.

The NIH’s decision came two weeks after the Fish and Wildlife Service called for protection of all chimpanzees as endangered. Until now there was a “split listing” that labeled wild chimps as endangered but those in captivity as threatened.