LAWYERS CHOOSING juries are allowed to reject anyone for cause -- the defendant's brother-in-law, for example, or the victim's mother. In addition, lawyers are allowed a fixed number -- usually 15 or 20 -- of so-called peremptory challenges: rejections that don't have to be explained. A prosecutor may believe a prospective juror doesn't seem smart enough to decide a complicated antitrust case; a defense lawyer may not want a juror who wears a flag lapel pin for a case involving anti-war protesters. A lawyer can object to such a juror without giving any reason for his decision.

What happens, though, if it appears that an attorney is systematically using his peremptory challenges to exclude all blacks from a jury -- or all whites? Twenty years ago, the Supreme Court held that such a practice was not a violation of the equal protection clause in an individual case, though this might be so "if a prosecutor in a county, in case after case, whatever the circumstances, whatever the crime and whoever the defendant or victim may be, is responsible for the removal of" persons of one race from juries. That kind of systematic discrimination doesn't happen much anymore. But in individual cases the practice of using peremptory challenges to exclude jurors of one race still exists.

Last week the U.S. Court of Appeals for the Second Circuit broke ranks with other federal tribunals on this matter. The court, which sits in New York, held that in any individual case where there is a "substantial likelihood that race is a motivating factor" in the prosecutor's use of his challenges, the defendant's Sixth Amendment right to an impartial jury has been violated. This decision, if it is appealed, will give the Supreme Court an opportunity to reconsider its earlier ruling -- at least five justices have said they would like to do this -- and limit a practice that sometimes allows racially segregated juries. California, Massachusetts, Florida and New Mexico already prohibit this kind of discrimination, and courts in those states have had no difficulty with the procedure. It has not led to mandatory racial and ethnic quotas on juries -- which would be an abuse -- but it has curtailed the kind of blatant exclusion that sometimes brings into question the validity of a jury's verdict. The New York panel is on the right track, and the Supreme Court should take this opportunity to reexamine its earlier decision in light of jury selection abuses that continue.