The State Department warned yesterday that a bill to end diplomatic immunity for relatives and support staff of foreign diplomats who commit serious crimes would "open the floodgates" to retaliation by other countries against U.S. diplomats and their families abroad.

"Regardless how grievous these matters may be, our own national interest must take precedence over any other consideration," Selwa Roosevelt, the U.S. chief of protocol, told the Senate Foreign Relations Committee.

" . . . If we unilaterally alter our treaty obligations, we surely will invite more harmful reciprocal action," she said.

Sen. Jesse Helms (R-N.C.), the bill's sponsor, argued that it was time to close "legal loopholes" that have enabled members of the foreign diplomatic community to escape punishment for serious crimes.

But in the face of staunch opposition from State Department officials and committee Chairman Claiborne Pell (D-R.I.), Helms signaled a willingness to soften his measure.

As many as 37,000 foreign diplomats, administrative and technical staff members and their families are immune to criminal prosecution in the United States under terms of the Vienna Convention. Diplomatic immunity has been a perennial problem in Washington and New York, involving everything from traffic and parking violations to assault and rape.

An attorney for Margaret Curry of New York testified yesterday that Curry was seriously injured July 12 when she was struck by a car driven by a youth who said he was a member of the diplomatic corps. Curry was directing a friend into a parking space when the youth, driving a Lincoln owned by the Afghan Consulate, insisted he needed the space, attorney Max D. Leifer said.

"Miss Curry advised him that the space was unavailable, at which point the gentleman engaged the vehicle and rather than backing away from the parking space, moved forward, striking Miss Curry . . . ," Leifer said. A passenger in the Lincoln told Curry "you look okay" before the car sped off.

Curry suffered a severe concussion and numerous bruises and has subsequently required psychiatric care, said Leifer, who added that New York police have declined to pursue the case because of the "diplomatic situation."

In other testimony, Kenneth W. Skeen, a former bouncer, told of being shot three times by the son of the Brazilian ambassador to the United States at a Washington nightclub in 1982.

Steve Goldstein, a New York City real estate salesman, described a 1985 incident in which the then-ambassador from Mexico to the United Nations smashed Goldstein's car window and pointed a pistol at his head in a parking dispute. Peter Christiansen, a retired New York City police detective, told of a 1981 serial rape investigation that led him to the son of an attache at the Ghanaian mission.

In each case, the assailant was immune from prosecution.

Roosevelt said the Skeen shooting never would have occurred if the Justice Department had promptly notified the State Department of a prior incident in which Skeen's assailant was accused of assault. That incident would have been grounds for expulsion from the United States, she said.

The State Department learned of the prior assault in a Dec. 2, 1982, letter about the Skeen case from the U.S. attorney for the District at the time. "Had I known about the first offense . . . that guy would have been out so fast it would have made your head go round," Roosevelt said.