A California research team said today it decided to replace an infant's defective heart with the heart of a baboon even though no animal had survived more than 5 1/2 months during experiments with the procedure.

Dr. Leonard L. Bailey, speaking to a packed news conference as animal rights activists demonstrated outside, said he knew that the risk of fatal rejection of the baboon heart was high but felt that the transplant offered 2-week-old "Baby Fae" her only chance to live.

The unusual experiment in this town 60 miles east of Los Angeles has sparked new debate among doctors and experts on medical ethics over the suffering by humans and animals involved in such procedures.

Hospital officials posted security officers with dogs outside today's news conference and put the baby's room under unusual security as about 30 members of People for the Ethical Treatment of Animals carried picket signs outside.

Bailey and his medical team at Loma Linda University Medical Center replaced the heart of the infant, who suffered from hypoplastic left heart syndrome, with the healthy heart of an 8-month-old female baboon Friday morning. Two days after the operation, the child, born one month premature with the left side of her heart almost entirely missing, was "beautifully healthy" and "doing everything right at the moment," Bailey said.

The disease, which is nearly always fatal within a few weeks, afflicts one of every 12,000 live babies born in the United States -- about 300 each year. Bailey said 3 1/2 years is the longest period he knows of a child surviving with the disease. A Philadelphia doctor has tried making temporary repairs in the deformed infant hearts, he said, but at last report had kept only two of 13 infants alive.

"We oppose the killing of a perfectly healthy baboon," said Lucy Shelton, southern California coordinator for the animal rights group demonstrating here. "We feel all they are doing is prolonging the suffering of the child."

Nearby, three or four demonstrators supporting the hospital also carried signs. "I think that preserving human life is the greatest thing," said Cheryl Harrison, who described herself as a Loma Linda housewife whose children have been cared for at the hospital.

The hospital has kept secret the identity of the baby who got the transplant and her family at the parents' request, saying only that they live in southern California. Bailey said that "after 14 months of agonizing" the Seventh-Day Adventist medical facility decided to fund five experimental baboon heart transplants into otherwise healthy human infants with fatally defective hearts. Baby Fae was the first infant brought to their attention who fit the criteria.

Bailey was joined at the news conference by Sandra Nehlsen-Cannarella, a New York immunologist called in to advise on ways to avoid rejection of the animal heart, and by Dr. Jack W. Provonsha, who holds doctorates in medicine and Christian ethics and directs the university's center for Christian bioethics.

Provonsha said a special university ethics panel determined that "all the ethical bases had been touched" and advised Bailey to proceed with the operation if he thought it had a reasonable chance to help the baby. Asked if he thought the parents believed the transplant could save their child or just wanted to help advance medical science, hospital spokesman Dick Schaefer spoke of "the look of hope" in the eyes of the baby's mother during the operation. When the baby, who had been near death, emerged pink and seemingly much healthier after the operation, the mother "was beaming from ear to ear. She had a big smile on her face and was extremely happy," he said.

Bailey, 41, a Takoma Park, Md., native who has performed more than 150 animal heart transplants here in the last seven years, gave a long, technical explanation of his experiments leading up to the operation and the complex procedures used to select from the medical center animal care facility a baboon with tissues most closely approximating those of the infant.

The baboon heart is nearly identical to a normal human infant heart, he said, and so far is pumping blood through the child's body at a normal rate. He said the baby was breathing easily and would be weaned soon from an oxygen-supplying ventilator.

Bailey said he has transplanted a goat's heart into an infant goat that matured normally and had kids. He said he thought the baboon heart could grow naturally inside the child's body, but officials here also have mentioned the possibility of a human-to-human transplant for the child. Suitable human heart donors for newborn infants are almost impossible to find, doctors said.

Bailey said his most successful transplant between species involved placing a lamb's heart in a goat. The goat died after 165 days when its body rejected the transplant, but Bailey said he was encouraged to try an ape-to-human transplant because evidence indicated that those species are more compatible than sheep and goats.

The medical team decided to use female baboons as transplant donors because they grow more slowly than males and their hearts would not be too large to use after a series of tissue- and blood-typing tests.

Nehlsen-Cannarella said sophisticated tests of infection-fighting substances in the blood, including mixing Baby Fae's white blood cells with those of six potential baboon donors, determined that the tissue of one young ape carried a significantly smaller risk of rejection by the infant's natural defense mechanisms.

The operation also was made possible by the Swiss discovery in 1972 of an immunosuppressive agent, cyclosporin-A, that did not leave the transplant recipient's body susceptible to major infections.

Experiments here also indicated that the transplant would not prove as harmful to infant kidneys as it had in adults. Bailey listed a number of normal infection-fighting systems that caused transplant rejection in adults but were far weaker in infants, increasing the chances of success with a newborn recipient.

Bailey said he perceived a need for animal transplants to help doomed infants while doing a residency in cardiovascular surgery at the Hospital for Sick Children in Toronto 10 years ago. His intention to conduct research on the problem drew "snickers" he said.

"I felt rather lonely while researching this for some years," he said. Medical journals rejected his articles; foundations turned down grant requests. Much of the $1 million spent on his experiments over the last seven years came from a special research fund using a share of fees earned by doctors at the 546-bed medical center.

Asked about criticism from animal rights activists, Bailey said he had pets and was a "supporter of animal rights, . . . but I am a member of the human species and I have to deal with helping the human species survive."

University spokesman Edward Wines said the heavy security at the news conference and the baby's intensive care ward was a simple precaution. "We really did not know what to expect," he said.