A new, single operation that may offer thousands of infertile women an alternative to trying for a "test tube baby" was announced yesterday by researchers at the National Institute of Child Health and Human Development.

Remarkably successful in monkeys, the operation involves harvesting a female's fertile egg just before it is due to be released from the ovary, and transferring it to the far end of her fallopian tube, near where the tube opens into the uterus.

Fallopian tube blockages -- whose causes include infection, sterilization operations or a previous ectopic [abnormally placed] pregnancy -- are the most common reason for female infertility, affecting more than 500,000 American women according to one estimate. Surgeons can successfully repair blockages in only about half the cases.

If the pregnancy rate achieved in monkeys is duplicated in humans, the researchers said, the new operation could become a leading treatment for some of these women, many of whom could otherwise become pregnant only by in vitro [test tube] fertilization.

The researchers have not tried the new technique in humans, and estimate that they will need to do up to two more years of animal experiments before they will be ready to do so. But Dr. James Sidbury, scientific director of the institute, predicted that the operation will prove safer than test tube fertilization.

Dr. Gary D. Hodgen, who directed the research, emphasized that researchers do not yet know whether the procedure will work safely in women. Although none of the monkeys had a pregnancy that developed abnormally in a fallopian tube, such pregnancies may be a dangerous complication of the operation in women, particularly those whose tubes are scarred by infection.

Under natural conditions, conception occurs when a ripe egg, released by one of a woman's two ovaries, is fertilized by a sperm as it travels down the fallopian tube towards the uterus. Once fertilized, the dividing egg implants in the wall of the uterus, which has developed a rich lining to receive it. Fallopian tube blockages cause infertility because they prevent sperm from reaching the egg.

Seeking a way to bypass blockages, Dr. Hodgen and Dr. Olivier Kreitmann of the institute's pregnancy research branch tried the new technique on 55 monekys whose fallopian tubes had been blocked surgically.

They measured hormone levels in the blood to predict the day the monekys would ovulate, or release a fertile egg. On the preceding day they mated them, and within 12 hours of the predicted time of ovulation they examined their ovaries with a laparoscope, an instrument used to view organs through a tiny incision in the abdomen.

In 40 monkeys, ovulation had not yet occurred and the ovaries contained a ripe egg. The researchers tried to aspirate the egg through a needle, and succeeded in 31 cases. In each, they then reinjected the egg into the fallopian tube near the uterus, bypassing the blockage.

Five monekys -- 16 percent -- became pregnant, and all five had apparently normal infants. By comparison, Hodgen pointed out that a fertile woman who has intercourse without contraception has a 15 to 20 percent chance of getting pregnant during a single menstrual cycle.

The technique needs to be refined. Measuring hormone levels does not allow researchers to time ovulation with enough accuracy to reliably harvest eggs two to four hours before they are released, he said. Harvesting an egg a few hours too early can prevent it from being fertile. In addition, harvesting a ripe egg can damage surrounding cells, which must produce the hormone progesterone after ovulation to sustain an early pregnancy. The researchers believe such factors impaired their success rate in monkeys.

Even if the operation proves successful in women, not all women with blocked tubes will be candidates. Hodgen said success depends on having a healthy section of fallopian tube near the uterus at least on one side.

"There are many women [who] have no severicable tube remaining on either side," he said.

Nevertheless, Sidbury said, the operation comes closer to duplicating the normal events of conception than in vitro fertilization, and eliminates some of the ethical problems. Doctors performing it do not have to choose one egg out of several, and do not have to examine embryos and decide whether they are viable.

"It [the new procedure] is what nature would have done anyway," he said.