ASHINGTON, June 14 - In an important ruling on sexual harassment in the workplace, the Supreme Court on Monday set guidelines for assessing an employer's liability for working conditions that become so unendurable as to lead a reasonable employee to resign without formally being dismissed.

The court held that an employer could ordinarily defend itself in such a situation by showing that it had adequate procedures in place for reporting harassment but that the employee failed to use those procedures.

But that defense is unavailable, Justice Ruth Bader Ginsburg wrote for the 8-to-1 majority, if a supervisor or manager had engaged in an "official act" like a demotion or reduction in pay that contributed to the intolerable work environment. In such a case, Justice Ginsburg said, the employer is liable, without any defense, as long as the facts of the case are proven.

The decision set aside a ruling by the federal appeals court in Philadelphia that, in a strongly pro-plaintiff approach at variance with that of some other courts, had ruled that a work environment so infected with sexual harassment as to induce an employee to quit was tantamount to an official firing. A company where such an incident occurred, even one with an anti-harassment policy, could not defend itself by showing that the employee failed to use that policy, the appeals court said.

That ruling had been appealed to the Supreme Court by the Pennsylvania State Police, which had been sued for damages by a police dispatcher, Nancy Drew Suders, who alleged that a work environment infused with sexual harassment had forced her to resign. The appeals court said that a voluntary resignation, as opposed to a dismissal, should never be seen as the kind of official act that strips an employer of a defense that is otherwise available.

With only Justice Clarence Thomas dissenting, Justice Ginsburg's opinion rejected both extremes, instead mirroring the Supreme Court's treatment of sexual harassment in a pair of cases in 1998. The court then for the first time addressed the standard for determining an employer's liability for sexual harassment carried out by a supervisor.

If the harassment culminated in a "tangible employment action" such as a discharge or demotion, the court said then, the employer was strictly liable, without the availability of any legal defense. But if the harassment led to a hostile environment without an adverse official action, the employer could defend itself by showing that it had set up a system for reporting and correcting sexual harassment but that the employee had unreasonably failed to use that system.

Those decisions, interpreting Title VII of the Civil Rights Act of 1964, the basic federal law against employment discrimination, led to the creation of anti-harassment policies in many workplaces. But they did not address the precise situation before the court in the Pennsylvania case, that of a "constructive discharge," a concept in labor law that applies when an employee has not been officially dismissed but has taken the reasonable step of resigning in the face of unendurable working conditions. Justice Ginsburg described the current case as "harassment ratcheted up to the breaking point."

Noting that a constructive discharge "may or may not involve official action," Justice Ginsburg said that "when an official act does not underlie the constructive discharge," the employer should have a chance to defend itself under the approach the court established in 1998. Her opinion vacated the appeals court's ruling and ordered that court to consider whether the defense should be available to the state police in this case, Pennsylvania State Police v. Suders, No. 03-95.

In his dissenting opinion, Justice Thomas said the court had adopted too open-ended a definition of a constructive discharge. He said a proper definition should require the employee to show some intent on the part of the employer to force a resignation.

These were among the day's other developments.

Retaliation

The justices agreed to decide whether the federal law against sex discrimination in education, usually referred to as Title IX, permitted a lawsuit alleging that a complaint of a Title IX violation had provoked purposeful retaliation.

In this case, Jackson v. Birmingham Board of Education, No. 02-1672, a man who coached girls basketball in the Birmingham, Ala., schools complained to his supervisors that the girls team was receiving fewer resources than was the boys team. The coach, Roderick Jackson, then began getting negative evaluations and was eventually relieved of his coaching duties.

Mr. Jackson brought a lawsuit alleging that he was the victim of unlawful retaliation under Title IX. The statute itself does not mention retaliation as a violation, and two lower federal courts dismissed his complaint on the ground that it had no legal basis. His appeal presents several interrelated questions: whether retaliation violates Title IX, whether someone alleging retaliation can bring a private lawsuit and whether someone who complains of but is not the direct victim of sex discrimination can bring such a suit.

Tax Challenge

The court ruled by a vote of 5 to 4 that the federal courts had jurisdiction to hear constitutional challenges to state tax credits. The decision, affirming a federal appeals court's ruling, permitted a suit to proceed against an income tax credit that Arizona makes available for contributions to a fund that subsidizes tuition at private schools, including religious schools. The plaintiffs, Arizona taxpayers, argued that the tax credit for religious school tuition violated the separation of church and state.

The question for the justices was whether this lawsuit fell within the category of lawsuits barred from federal court by the Tax Injunction Act. That law prohibits the federal courts from restraining "the assessment, levy or collection of any tax under state law." The statutory question was whether a tax credit fell within the definition of "assessment." Arizona's appeal, Hibbs v. Winn, No. 02-1809, also raised a deeper issue of federalism, arguing that the law's purpose was to bar federal court interference with state tax systems.

The case was potentially important, because the Supreme Court has regularly decided cases on discriminatory state taxation, finding no barrier in the Tax Injunction Act. That "decades-long understanding" should allow this suit to proceed, Justice Ginsburg wrote for the majority.

In a dissent, Justice Anthony M. Kennedy said the court should have deferred to the state on federalism grounds. Justice Kennedy evidently had originally been designated to write a majority opinion for this position but lost his majority along the way. Of 10 cases argued in January, this was Justice Ginsburg's third majority opinion, while only Justice Kennedy has no majority opinion from that month. Justices John Paul Stevens, Sandra Day O'Connor, David H. Souter and Stephen G. Breyer voted in the majority.