A bogus letter informing a blood donor that he had been exposed to the AIDS virus may have been an office prank, according to the local chapter of the American Red Cross.

The unidentified man had donated blood at his office, and then received a letter bearing a Red Cross logotype at his home in mid-May, saying that his blood tested positive for the HIV virus. The man, who became distraught and even considered suicide, called the Red Cross to verify the information, which proved to be false, said Dr. Fred W. Darr, medical director of the Washington region of the American Red Cross.

"We don't send out letters like that," Darr said yesterday. Such information is delivered in person, he said, explaining that if a donor has a positive HIV test, a general letter is sent to the person by registered mail asking the individual to visit the office.

Darr said an internal investigation did not indicate that any Red Cross employe was involved in the hoax. Darr said organization lawyers are looking into the possibility that the letter had been sent by a coworker of the man or someone else who knew he had donated blood. The letter was not sent on genuine Red Cross stationery.

The local Red Cross collects blood from about 800 donors each week. Each unit of blood is tested for AIDS antibodies, syphilis and hepatitis. Forty-five donors here tested positive for AIDS this year, Darr said.