More than 500 doctors, researchers and public health specialists are calling on the Food and Drug Administration to eliminate constraints on blood donations by gay and bisexual men, saying the agency did not go far enough when it relaxed its restrictions earlier this month.

The demand was made in a letter dated Thursday that was written by two doctors at the University of California, San Francisco, and signed by hundreds of medical professionals at places like Emory University, Harvard Medical School and the University of Florida. Dispensing with the rules, the letter said, would help to address a drastic drop in the blood supply during the coronavirus pandemic.

The F.D.A.’s restrictions date to 1983, early in the AIDS epidemic, when the agency banned donations from gay and bisexual men outright for fear of introducing H.I.V. into the nation’s blood supply. In 2015, the agency scrapped the lifetime ban but continued to bar donations from men who had had sex with men in the previous 12 months, arguing that the waiting period was necessary to keep the blood supply safe.

On April 2, after donations dropped precipitously as blood drives were canceled nationwide because of the coronavirus, the agency shortened the deferral period to three months from one year, saying that recent studies had shown that doing so would not compromise the safety of the blood supply.