PITTSBURGH (AP) _ A pioneering surgeon defended the world's first baboon-to-human liver transplant against criticism from animal-rights activists, saying doctors' first obligation is to human life.

''Our passion and our commitment is to human beings. We took an oath when we graduated from medical school to do that,'' said Dr. Thomas Starzl, a University of Pittsburgh transplant specialist who helped obtain approval from a hospital board for the operation.

About 15 people demonstrated outside Presbyterian University Hospital on Tuesday, chanting, ''Animals are not spare parts 3/8''

''We don't believe you should sacrifice one species for another. No one asked the baboon what he thought,'' Irene Simko said.

A 35-year-old man, whose name was withheld at his request, underwent the transplant at the hospital on Sunday. He was dying from recurring infections of hepatitis B, which destroys the liver. He was in critical condition today.

Doctors said that if the man had been given a new human liver, the hepatitis virus probably would have damaged that, too. Doctors believe baboon livers are not susceptible to the virus.

A 15-year-old, laboratory-bred baboon was killed to provide the organ. Hospital spokeswoman Lisa Rossi said the animal was anesthetized first.

Doctors said baboon-to-human transplants could save lives by easing a shortage of organs for people needing new livers.

''I admire anyone with passion. It takes a lot of passion and commitment to be out there as they are,'' Starzl said of the demonstrators.

But he added, ''Our passion and our commitment is to human beings. Our ultimate fidelity really has to be to our patients.''