The Supreme Court on Thursday rejected school diversity plans that take account of students' race in two major public school districts but left the door open for using race in limited circumstances.

The decision in cases affecting schools in Louisville, Ky., and Seattle could imperil similar plans in hundreds of districts nationwide, and it further restricts how public school systems may attain racial diversity.

The court split, 5-4, with Chief Justice John Roberts announcing the court's judgment. The court's four liberal justices dissented.

Yet Justice Anthony Kennedy would not go as far as the other four conservative justices, saying in a concurring opinion that race may be a component of school district plans designed to achieve diversity.

To the extent that Roberts' opinion can be interpreted to foreclose the use of race in any circumstance, Kennedy said, "I disagree with that reasoning."

He agreed with Roberts that the plans in Louisville and Seattle violated constitutional guarantees of equal protection.

The two school systems in Thursday's decisions employ slightly different methods of taking students' race into account when determining which school they will attend.

Federal appeals courts had upheld both plans after some parents sued. The Bush administration sided with parents, arguing that racial diversity is a noble goal but can be sought only through race-neutral means.

Louisville's schools spent 25 years under a court order to eliminate the effects of state-sponsored segregation. After a federal judge freed the Jefferson County, Ky., school board, which encompasses Louisville, from his supervision, the board decided to keep much of the court-ordered plan in place to prevent schools from re-segregating.

The lawyer for the Louisville system called the plan a success story that enjoys broad community support, including among parents of white and black students.