The Supreme Court justices, hearing arguments on school integration, signaled yesterday they are likely to bar the use of race when assigning students to public schools.

Such a ruling could deal a blow to potentially hundreds of school systems across the U.S. that use racial guidelines to maintain a semblance of classroom integration in cities whose neighborhoods are divided along racial lines.

However, it would be a major victory for those who have called for "colorblind" decision-making by public officials.

Yesterday's argument also might mark the emergence of a five-member majority determined to outlaw the official use of race guidelines in schools, colleges and public agencies.

"The purpose of the Equal Protection clause is to ensure that people are treated as individuals rather than based on the color of their skin," Chief Justice John Roberts Jr. said.

Three years ago, the court upheld affirmative action at colleges and universities. But that 5-4 decision depended on the now-retired Justice Sandra Day O'Connor. Since then, President Bush's two appointees - Roberts and Justice Samuel Alito Jr. - have joined the court, and the tenor of yesterday's debate suggested a new majority would frown on race-based affirmative action.

At issue were the guidelines adopted by school boards in Seattle and Louisville, Ky. The two cases are Parents Involved in Community Schools v. Seattle School District No. 1 and Meredith v. Jefferson County Board of Education.

Seattle allows its students to chose which high school they want to attend but tries to maintain a racial balance within 10 percentage points of its overall enrollment. In 2001, before the program was suspended, 210 white students and 90 minorities were denied their first choice of a high school.

The Louisville schools seek to keep black enrollment between 15 percent and 50 percent.

Both policies were challenged by parents of a small number of students, most of them white, who were denied their first-choice school because of their race.

School officials could not say how many districts use racial guidelines that could be affected by the court's ruling. But a ruling against such policies could put at risk many magnet school programs that use race as an admissions factor.

The justices who spoke during the argument all agreed racial integration is a laudable goal. However, a narrow majority - in comments, questions and past decisions - made clear their belief that the Constitution forbids shifting children from one school to another based on their race.

Until yesterday, civil-rights lawyers held out the faint hope that Justice Anthony Kennedy, a centrist, might vote to uphold local school integration plans, even though he regularly had opposed race-based affirmative action.

But Kennedy quickly dashed those hopes. He told a lawyer for the Seattle school board that "outright racial balancing . . . is patently unconstitutional. And that seems to be what you have here."

Agreeing with Kennedy, Roberts noted that the districts were making decisions on assigning students to schools "based on skin color and not any other factor."

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