Dr. Rosalyn S. Yalow, 89, a medical physicist who persisted in entering a field largely reserved for men to become only the second woman to earn a Nobel Prize in medicine, died Monday in New York City, where she had lived most of her life.

Dr. Yalow, a product of New York City schools and the daughter of parents who never finished high school, graduated magna cum laude from Hunter College at age 19 and was its first physics major. Yet she struggled to be accepted for graduate studies. In one instance, a skeptical Purdue University wrote: "She is from New York. She is Jewish. She is a woman."

Undeterred, she went on to carve out a renowned career in medical research, largely at a Bronx veterans hospital, and in the 1950s became a codiscoverer of the radioimmunoassay, an extremely sensitive way to measure insulin and other hormones in the blood. The technique invigorated the field of endocrinology, making possible major advances in diabetes research and in diagnosing and treating hormonal problems related to growth, thyroid function, and fertility.

The technique "brought a revolution in biological and medical research," the Karolinska Institute in Sweden said in awarding Dr. Yalow the Nobel Prize in physiology or medicine in 1977.

Dr. Yalow developed radioimmunoassay with her longtime collaborator, Dr. Solomon A. Berson. He died in 1972, before Dr. Yalow was honored with the Nobel. The institute does not make awards posthumously. - N.Y. Times News Service