PITTSBURGH (AP) _ Hundreds of organ transplant recipients converged at a medical conference to celebrate life Friday, while medical experts struggled with the question of which dying patients should receive scarce donor organs.

''It's like a great big birthday party,'' said Margaret McStay of Dublin, Ireland, whose son Colin, 4, received a new liver here three years ago.

Colin played with a stuffed penguin named Oliver, which he calls ''O'Liver.''

''Just the scale of this is almost frightening,'' Mrs McStay said of the 400 organ recipients and their families at the International Organ Transplant Forum.

Some, like the McStays, came to share their joy and triumphs. Others, like Shirley Gailey of New Brighton, Pa., came for advice dealing with life after a liver transplant.

Ms. Gailey is unemployed and said prospective employers claim they'll lose their group insurance policies if they hire her.

''It's like ... the plague or something,'' said Ms. Gailey, 51, who received her new organ in 1985. ''I sure hope somebody has some answers.''

While the transplant recipients released 600 purple helium-filled balloons, each carrying an organ donor card, medical experts spent much of the day looking for answers on how best to allocate donor organs.

More than 12,000 people are awaiting organ transplants in the United States, according to the Pittsburgh Transplant Foundation.

While more transplants than ever are being performed, ''for every person you transplant you get one back on the waiting list,'' said foundation director Brain Broznick.

''It's intrinsically unfair'' to award organs based on a patient's ability to pay or to attract public attention, said Dr. Olga Jonasson of the University of Illinois.

To ensure fairness, the nation's approximately 200 transplant centers will be required beginning Oct. 1 to choose organ recipients based on a point system. Failure to do so will result in the loss of federal Medicare reimbursements.

The computerized system, developed by Dr. Thomas E. Starzl of the University of Pittsburgh, weighs medical urgency, waiting time, distance between donor and recipient and the quality of organ and recipient tissue match.

In addition, transplant centers, organ procurement agencies and tissue- typing laboratories nationwide will be required to share kidneys considered near-perfect matches. Such organs comprise fewer than 5 percent of all kidneys donated for transplant. Nearly 9,000 kidneys were transplanted in the United States last year.

While few experts question the need for an improved system, many question whether computer ratings are the best method.

''It's an intensely human set of transactions,'' said Dr. Paul Russell, a Harvard University surgery professor.

''We may come to find we're in an Orwellian situation where we're being forced to do things that might not be correct,'' said Dr. Frederick Merkel, medical director of the Chicago Regional Organ and Tissue Bank. ''There are so many factors that change. Without flexibility, I think we'll find we're losing more organs than we should.''

The federal guidelines are expected to clash with laws in some states reserving donated organs for state residents, said Eugene Peirce, executive director of the United Network for Organ Sharing in Richmond, Va.

The state laws were enacted before the U.S. Department of Health and Human Services hired that agency to develop a national organ network and set guidelines for sharing organs.

The Defense Department also has passed regulations giving military personnel and their families preference for organs donated by other service members, according to Peirce.

Peirce and other experts agree much remains to be resolved, although Jonasson said the new rating system is ''a step at least in the right direction.''