The Food and Drug Administration announced on Tuesday that it would scrap a decades-old lifetime prohibition on blood donation by gay and bisexual men, a change that experts said was long overdue and could lift the annual blood supply by as much as 4 percent.

The F.D.A. enacted the ban in 1983, early in the AIDS epidemic. At the time, little was known about the human immunodeficiency virus, which causes the disease, and there was no quick test to determine whether somebody had it. But science — and the understanding of H.I.V. in particular — has advanced in the intervening decades, and on Tuesday the F.D.A. acknowledged as much, lifting the lifetime ban but keeping in place a block on donations by men who have had sex with other men in the last 12 months.

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F.D.A. to Lift Lifetime Ban on Blood Donations by Gay Men