The F.D.A. initially banned gay men from giving blood when the risk of AIDS from transfusions was recognized. At the time, it was considered the best way to keep the blood supply safe. The restriction applies to any man who acknowledges having had sex with another man since 1977.

Gay rights groups, blood banks, and many doctors and scientists consider the ban inconsistent with the restrictions placed on other high-risk groups and antiquated in light of the advances that have been made in testing donated blood.

Donated blood is routinely screened for H.I.V. and other infectious agents. The F.D.A. employs multiple safeguards, including donor evaluations and computerized blood testing, to ensure that infected blood is not distributed.

Since the 1980s, when the ban was enacted, the tests have become much more sensitive and accurate. The F.D.A. did not take this into account when it reviewed the restriction in past years, said Bebe Anderson, H.I.V. Project Director for Lambda Legal, a national gay rights group that has opposed the ban.

“We think this policy is harmful and makes no sense,” she said. “Turning away perfectly good donors gives an incorrect and harmful message about transmission risk. This is screening donors based on sexual orientation, not on risk. It also stigmatizes people who wish to donate.”

Brad Baso, a project manager for a nonprofit organization in St. Paul, first tried to donate blood in college but was turned away because he is gay. Now, he finds himself trying to explain to co-workers why he can’t donate blood when blood drives are held at work.

“I work in the nonprofit sector, and it’s important for me to give back. But when you can’t participate in a workplace event, it makes you feel bad,” said Mr. Baso, 31. “I have to go into a teachable moment and explain that my government thinks my blood is tainted.”