The US Supreme Court has made a modest but important exception to the principle it had held for 70 years - that no evidence obtained illegally could be used in court. The court's new ruling is sound: If the evidence would have been turned up legally anyway, it can be admitted.
The decision represents the dropping of only one shoe. In the next month the court is expected to rule on a broader aspect of illegal evidence and that is whether it can be used in a trial if the law-enforcement or judicial officials who obtained it were unaware they were making a mistake - the so-called ''good faith'' issue.
The court's Monday ruling comes against a backdrop of a changing mood in America toward crime in recent years. It has swung from concern for the rights of the accused to attention on the rights of victims. There has been increasing sentiment in American society, as well as among political leaders, for swifter and stiffer punishment of the guilty - and increasing displeasure that some persons who did in fact commit crimes escaped conviction through technicalities.
The court's ruling will have little immediate impact on the operation of many lower courts and police. Illegally obtained evidence that would have surfaced anyway has been used increasingly in court cases.
Yet the Supreme Court's approval of this practice offers a challenge to the hard-pressed American law-enforcement system: not to substitute illegally gathered information for hard investigative work.
Until future cases yield firm guidelines it will be difficult for police to know what kinds of illicitly obtained evidence will be judged admissible in court and what will be ruled out. To cope with these problems many police need additional training in legal issues and in professional methods that incorporate the latest technology.
The challenge to law enforcement would be redoubled if the Supreme Court were to allow courts to consider evidence obtained in good faith even if it were discovered illegally.
Police then would find it in their interest to be uninformed on fine points of the law. However, it is in society's best interests for police to continue to increase their knowledge of the law, thereby improving the quality of work they are able to do. In addition, some law-enforcement officers might be tempted to claim they knew less law than in fact they did.
Courts at all levels now should rigorously examine requests to admit evidence obtained illegally to make certain that it would have been found anyway. This exception to the concept of excluding illegal evidence should be used when necessary, to protect victims and society at large. But it also is important that neither police nor prosecutors take advantage of the ruling. The accused still has rights.