It is not necessarily unlawful for an employer to adopt policies that put older workers at a disadvantage. Such policies pass muster under the Age Discrimination in Employment Act as long as they are based on ''reasonable factors other than age.''

The question in a Supreme Court argument on Wednesday was whether the employer has to prove that such ''reasonable factors'' exist, or whether it is up to the employee who has brought a lawsuit to show that they do not.

The burden of proof makes a substantial difference in any lawsuit, although statutes rarely specify which side bears it. For federal laws against race and sex discrimination in the workplace, the Supreme Court has filled the gap by developing fairly elaborate procedures that plaintiffs and defendants must follow. But for age discrimination, the rules have remained murky, leaving the lower courts in confusion over how to handle this rapidly growing category of workplace discrimination claims.

The argument the justices heard on Wednesday was in a case brought by two dozen workers at a federal research laboratory in upstate New York. Carrying out a reduction in force, the employer, Knolls Atomic Power Laboratory, which is owned by the Lockheed Martin Corporation, terminated 31 employees after using a set of guidelines to evaluate workers' skills and amenability to retraining. All but one dismissed employee was over 40, the age at which the protections of the federal age discrimination law begin to apply.

Most of the affected employees joined a lawsuit arguing that there was no justification for using an evaluation system that had such a starkly disparate impact on older workers, and that the procedure consequently violated the federal law. The plaintiffs won in a jury trial. But the judgment was overturned by the United States Court of Appeals for the Second Circuit, in Manhattan, which held that plaintiffs in such a case had the burden of showing that the policy they were challenging was unreasonable.

In the Supreme Court, the Bush administration supported the employees' appeal, Meacham v. Knolls Atomic Power Laboratory, No. 06-1505. Daryl Joseffer, an assistant to the solicitor general, told the justices that the inclusion in the statute of ''reasonable factors other than age'' as an exception to liability showed that Congress was offering employers a defense, the existence of which they would have to prove.

''But even if the text wasn't so clear, one would logically put it on the employer,'' Mr. Joseffer continued. ''All else being equal, the employer is in a better position to explain the reasonableness of its very own business practice.''

The employees' lawyer, Kevin K. Russell, made a similar point, and met with resistance from both Justice Anthony M. Kennedy and Chief Justice John G. Roberts Jr.

''That doesn't seem a very compelling case,'' the chief justice said, because in pretrial discovery, employees' lawyers could question company officials about their rationale. ''It doesn't seem to me that the fact that the employer possesses the information, given the very liberal discovery we have, is much of a factor,'' he said.

The employer's lawyer, Seth P. Waxman, said the court should keep in mind the difference between age discrimination and other kinds of workplace discrimination. Policies that have a differential impact on the basis of race or sex can rarely be justified as reasonable, he said, while age, on the other hand, ''often does correlate with reasonable employment factors.''

Consequently, Mr. Waxman continued, ''the presumption actually is quite weak'' that a challenged policy amounts to improper age discrimination, and it made sense for plaintiffs to have to prove that there was no legitimate justification.

The justice most skeptical of Mr. Waxman's argument was Justice Ruth Bader Ginsburg, who questioned him closely both on his legal theory and on the facts of the case.

The argument in this case was the 70th and final one of the court's current term. From now until the term ends in late June, the justices will spend their time deciding the remaining cases they have heard and selecting new appeals to hear in the next term.

In a decision on Wednesday, the court held by a vote of 9 to 0 that a search by the police in connection with an arrest based on probable cause is constitutional, under the United States Constitution, even if the arrest was invalid under state law.

The case, Virginia v. Moore, No. 06-1082, was an appeal by the State of Virginia of a ruling by its Supreme Court. The police arrested a man for driving with a suspended license, and in the subsequent search found a small quantity of crack cocaine. Under Virginia law, driving with a suspended license merits only a summons, not an arrest, and the state court consequently found the search to be invalid under the Fourth Amendment.

In his opinion overturning that decision on Wednesday, Justice Antonin Scalia said that the Fourth Amendment, which prohibits unreasonable searches and seizures, was not intended by its framers ''as a redundant guarantee of whatever limits on search and seizure legislatures might have enacted'' in the individual states.

As long as ''an officer has probable cause to believe a person committed even a minor crime in his presence,'' Justice Scalia continued, ''the arrest is constitutionally reasonable'' even if it violates state law. States are free to give their citizens more protection than the constitutional minimum, he added, but ''when states go above the Fourth Amendment minimum, the Constitution's protections concerning search and seizure remain the same.''

The other members of the court all signed Justice Scalia's opinion except Justice Ginsburg, who concurred separately. She said she agreed that the arrest and search did not violate the Fourth Amendment, but found the issue more complicated, both as a matter of constitutional history and precedent, than Justice Scalia's opinion acknowledged.

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