The world's first test-tube twins were born here yesterday and Australian doctors quickly performed minor heart surgery on the ailing boy. They said they believe he will live.The girl was said to be howling healthly in the arms of the mother, Radmilda Mays, 31.

Amanda and her brother Stephen, weighing 5.5 and 4.95 pounds respectively, were born to Mays and her husband Richard, 33, an engineer.

Delivered by Caesarian section, Stephen and Amanda were the first twins known to have been conceived by artificial insemination. Stephen, who was delivered first, was born "blue" with a condition described as a transposition of the heart vessels.

Working quickly to try to save Stephen, doctors at Melbourne's Queen Victoria hospital performed a "balloon septostomy," a minor operation involving the insertion of a catheter into a vein and the inflation of a tiny balloon in the baby's chest to stabilize his heart. A hospital official said the operation was a temporary measure until Stephen, who was still listed in serious condition, is old enough for more complex surgery.

In vitro fertilization is a method in which the female egg is fertilzed with male sperm in a laboratory and then implanted in the mother's womb. In the case of the Mays' twins, two eggs were implanted in the hope that one would develop.