LEGISLATORS, prosecutors and ordinary folks are understandably furious when someone entitled to diplomatic immunity breaks the law here and is not punished. Some of the offenses committed not only by diplomats but by their families and staffs have been horrendous. Remember the drunken ambassador who hit several automobiles and severely injured a young man who had been sitting in one of them? Remember the Howard University professor paralyzed in an automobile accident caused by an uninsured diplomat? Such events usually produce a demand for an end to diplomatic immunity, but cooler heads prevail when the State Department explains again the reasons for immunity. It is granted not out of some naive assumption that diplomats do no harm or some generous belief that host countries must be nice to their guests. It is done, quite simply, to protect Americans serving in diplomatic posts abroad.

Earlier this year, Sen. Jesse Helms took testimony in the Senate Foreign Relations Committee from some of the victims of immunized crime and, in response, proposed legislation that would have severely limited the privilege in a way that would have created trouble for this country's Foreign Service. But he has rewritten that proposal in cooperation with the State Department and his Democratic colleagues on the committee. Last week, the Senate adopted compromise language. It puts into law some of the practices already being observed informally, and it requires a tough reponse when offenders cannot be prosecuted. From now on, when diplomats are charged with serious crimes, the State Department will ask their governments to waive immunity and will accept a refusal to waive only from a foreign minister. If immunity is insisted upon, the person charged will immediately be expelled from this country, and steps will be taken to ensure that he never returns. The Helms amendment also tightens up the definition of family members entitled to immunity by limiting the privilege to spouses and minor children. It also raises the amount of liability insurance required to be carried by diplomats from $300,000 to $1 million.

A surprising 58,000 people are entitled to diplomatic immunity in the United States. Practical steps to ensure that the privilege is not abused make sense. Sen. Helms' proposal, which does not restrict immunity in a way that would threaten the safety of American diplomats abroad, is a good one.