With blood donations steadily falling and demand for transfusions rising, the United States is headed for a blood supply shortfall next year, according to blood researchers.

Unless those trends are suddenly reversed, the usable blood supply in 2000 will drop by 600,000 pints to 11.69 million units--249,000 units fewer than blood banks and hospitals expect to need, according to the National Blood Data Resource Center in Bethesda.

"Blood banks are notorious for crying wolf again and again, and sometimes it's hard to take them seriously," said Arthur Caplan, a University of Pennsylvania bioethicist who chairs a federal advisory committee on blood safety and supply. "But if you look at the numbers, this is serious."

In 1992, the available supply of donated blood was 13.2 million pints, but surveys showed that it fell to 12.9 million in 1994 and 12.3 million in 1997. In 1992, there were 11.3 million transfusions. That fell slightly before increasing to 11.5 million in 1997.

The possible shortage comes as physicians are becoming more reliant on blood transfusions as they attempt an increasing number of organ transplants, complex surgical procedures and aggressive new cancer therapies.

"Blood truly is to the health care system as oil is to transportation, and you're looking at declines that aren't just seasonal," Caplan said. "That spells real trouble for every American. That means delays in elective surgery, problems in treatment of cancer and problems in the supply of blood products for people with very serious diseases."

Experts say the supply projections raise questions about whether Americans are less generous and community oriented than their parents were and whether long-held fears of contracting blood-borne illnesses continue to make people fear rolling up their sleeves.

It is clear it has become increasingly difficult to persuade companies to encourage their workers to give blood, according to Harvey G. Klein, the chief of transfusion medicine at the National Institutes of Health clinical center in Bethesda. "Now we have many more small businesses and some reluctance on the part of many employers to allow space and time for blood drives," Klein said. "The opportunity to collect is not what it was 15 years ago."

"What has alarmed me and should alarm everyone is that over the past several years there have been times when there simply wasn't enough blood of the right blood group to perform surgery that should be done but can be postponed," he said.

In contrast to the nation as a whole, the Washington-Baltimore area has seen increased donation in the past several years, said Patrick J. Smith, spokesman for the area's biggest blood collector, the Greater Chesapeake and Potomac region of the American Red Cross.

In 1997, the region collected 262,000 units of whole blood, compared with a projected 282,000 this year.

Despite that collection advantage, the local Red Cross group imports blood from other areas to meet the demands of a region dominated by major teaching and research institutions, Smith said.

The threat of a blood deficit increases pressures on government officials deciding how best to assure the safety of the blood supply.

An expert panel last week recommended that the Food and Drug Administration bar blood donations from Americans who visited Britain during the "mad cow" outbreak to reduce the chances of a tainted donation--even though there has never been a documented case of such an infection.

That policy could cut donations by as much as 286,000 units a year, depending on whether the ban extends to people who spent only a week or as much as a year in Britain.

A different advisory panel in April urged the FDA to stop requiring blood banks to destroy blood collected from people with hemochromatosis, a hereditary disease that overloads their bodies with iron and can cause irreversible organ damage.

Doctors say the best therapy is to drain a pint of blood from the patient about four times annually to reduce iron levels. Each year up to 300,000 pints taken from people with hemochromatosis are discarded because they are not considered altruistic donations.

Doctors say that as long as the blood is not contaminated by a virus, it would be perfectly safe for transfusion. But because hemochromatosis patients now pay for the bleeding procedure, called a phlebotomy, the American Red Cross views them as having an unacceptable financial motivation to be bled for free if they can donate acceptable blood. That could lead to lying about their lifestyles or medical conditions, officials said.

"We still feel voluntarism is the bedrock of safety," said American Red Cross Medical Director Richard J. Davey. "If there's a way that we can remove the financial incentives for those people to donate . . . we would welcome them as volunteer blood donors."

Several firms are working on blood substitutes that can carry oxygen or perform other limited blood functions that could ease demand for transfusions, while others are seeking ways to extend the shelf life of donated blood.

Other researchers expect to be able to eradicate all transmissible diseases from blood through genetic testing and manipulation.

That would dramatically increase the supply because blood banks would be able to "cleanse" hundreds of thousands of contaminated donations each year instead of destroying them.

Scientists are also finding ways to generate large volumes of scarce blood proteins by enabling genetically altered animals to produce them in milk, for example.

If that succeeds it also could take pressure off the demand side.