WASHINGTON, March 19--U.S. hopes for building a defense against the intercontinental ballistic missile were brightened today by the announced success of a series of high altitude atomic test explosions.

The hitherto secret launching of three rockets bearing nuclear warheads into space late last summer was described by the Defense Department as having "advanced the basis of knowledge required to develop anti-missile missiles."

Dep. Secretary of Defense Donald Quarles and his scientific assistants made it clear that the prime purpose of the experiments, which threw a thin curtain of radation around the earth for short periods, produced results that will be used in perfecting the radar systems needed to put an anti-missile missile on course.

Khrushchev's Tone Conciliatory

MOSCOW, March 19--Soviet Premier Nikita Khrushchev said today he is willing to open a foreign ministers conference on Germany May 11 but insisted only a summit meeting can brush away the threatening clouds of war.

"I am convinced there will be meetings of both the foreign ministers and heads of states," he told a news conference. He said he is waiting for the West to make concrete proposals.

Khrushchev again denied May 27 was intended as an ultimatum date. "If it is not settled in six months, let it be settled in seven," he said.