Doctors could save the lives of an estimated 1,100 newborn babies a year and prevent brain damage in others by giving an antibiotic to expectant women infected with a common bacterium, researchers said yesterday.

A new study found that ampicillin administered to infected women during childbirth significantly reduced the chances of their babies contracting group B streptococci, which can lead to permanent brain damage and sometimes death.

"What this study does is demonstrate that you can have a significant impact on the incidence of this disease," Dr. Kenneth M. Boyer, who headed the study at the University of Chicago Pritzker School of Medicine, said in a telephone interview. "I think this will have a significant impact on neonatal care," said Boyer, whose study was published in the New England Journal of Medicine.

Group B streptococci is often present in the reproductive tract and is usually harmless except when childbirth is involved. Two of every 1,000 babies in the United States become infected during birth and develop a variety of diseases, including pneumonia and meningitis.

Such diseases can cause mental retardation and other permanent deformities, and about 1,500 babies die in the United States each year due to the bacterium, Boyer said. Also, mothers may develop postnatal complications from it.

Another study in yesterday's New England Journal concluded that the quality of life varies considerably among people taking different blood-pressure medicines, and that doctors should consider drugs' side effects when deciding how to treat patients.

A major difficulty in treating high blood pressure, or hypertension, is to persuade patients to keep taking their medicines.

"Some patients perceive the use of antihypertensive medications to be more troubling than their seemingly symptomless disease," researchers wrote.

The doctors measured the effects of three common blood-pressure drugs: captopril, methyldopa and propranolol. They concluded that people tolerate captopril best and methyldopa least, while propranolol falls between.

The study was financed by Squibb Corp., which makes captopril.