ASHINGTON, June 29 — The Supreme Court on Tuesday rejected Congress's latest effort to curb children's access to sexually explicit material on the Internet. But at the same time it gave the Bush administration a second chance to defend the law as a trial on its constitutionality goes forward in Federal District Court in Philadelphia.

The 5-to-4 majority kept in place an order that the district court issued in 1999, blocking enforcement of the Child Online Protection Act until its validity can be resolved. The six-year-old law, which imposes criminal penalties of as much as $50,000 a day on commercial Internet sites that make pornography available to those younger than 17, has never taken effect.

The decision came on the final day of the Supreme Court's term. Justice Anthony M. Kennedy, writing for the majority, said that the government must now show why the voluntary use of filters to screen out material unsuitable for children would not work as well as the law's criminal penalties. Filters "impose selective restrictions on speech at the receiving end, not universal restrictions at the source," Justice Kennedy wrote.

The opinion, which was joined by Justices John Paul Stevens, David H. Souter, Clarence Thomas and Ruth Bader Ginsburg, suggested strongly that the government would not be able to demonstrate that the penalties were better than filters. Not only are filters less restrictive, but they "also may well be more effective," Justice Kennedy said, because they can block pornography from anywhere in the world, while the statute applies only to pornography posted on the Web from within the United States.

Even so, the court kept open the possibility that the law, known as COPA, might ultimately be upheld.

"This opinion does not hold that Congress is incapable of enacting any regulation of the Internet designed to prevent minors from gaining access to harmful materials," Justice Kennedy said.

He said the decision "does not foreclose the district court from concluding, upon a proper showing by the government that meets the government's constitutional burden as defined in this opinion, that COPA is the least restrictive alternative available to accomplish Congress's goal."

Under the court's First Amendment precedents, government-imposed restrictions must go no further than necessary to accomplish a "compelling government interest" - in this instance, protecting children from harmful material on the Internet. The government must show that it is using the "least restrictive means" to achieve its goal.

The coalition of Internet publishers and free-speech groups that filed suit to block the law have argued that the existence of filters showed that criminal fines and prison sentences were not the least restrictive approach. A year ago, the Supreme Court upheld a law that required public libraries to install Internet filters as a condition of receiving federal money.

In a dissenting opinion on Tuesday, Justice Antonin Scalia said the majority had subjected the Child Online Protection Act to too searching a constitutional review. He said that because the commercial pornography that is the law's target "could, consistent with the First Amendment, be banned entirely, COPA's lesser restrictions raise no constitutional concern."

The three other dissenters, Justices Stephen G. Breyer and Sandra Day O'Connor along with Chief Justice William H. Rehnquist, took a different approach. They said, in an opinion written by Justice Breyer, that the law should be interpreted to apply only to a narrow category of obscene material and should be upheld on that basis.

"Properly interpreted," Justice Breyer wrote, the law "imposes a burden on protected speech that is no more than modest," reaching only "borderline cases" beyond speech that is obscene and that thus lacks legal protection. Justice Breyer said that while the plaintiffs raised the specter that the law might apply to famous novels or serious discussions of sexuality, this was not the case. "We must interpret the act to save it, not to destroy it," he added.

Further, Justice Breyer said, there was little reason to suppose that filters would achieve the purpose of shielding children. He said the software "lacks precision" and depends for its effect on parents' willingness to pay for it, install it and monitor their children's computer use.

The court and Congress have had a tangled relationship on the question of Internet pornography. In 1997, the court unanimously invalidated Congress's first effort, the Communications Decency Act of 1996. Congress responded quickly by passing the Child Online Protection Act the next year, responding to a number of the court's concerns by defining pornography more precisely and limiting the reach of the statute to commercial Web sites.

The American Civil Liberties Union, which had organized the successful challenge to the first law, sued to block the new law as well, and won in both the Federal District Court and the United States Court of Appeals for the Third Circuit, in Philadelphia. The Third Circuit found then that the law's reference to "contemporary community standards" would give "the most puritan of communities" an effective veto over Internet content.

The Supreme Court, in a 2002 decision, disagreed with that analysis and sent the case back to the Third Circuit. This time, the appeals court ruled that the law did not meet the First Amendment's "least restrictive means" test. The Bush administration then appealed that ruling to the Supreme Court, leading to the decision Tuesday, Ashcroft v. American Civil Liberties Union, No 03-218.

Mark Corallo, a spokesman for the Justice Department, expressed the administration's dismay with the ruling. "Congress has repeatedly attempted to address this serious need, and the court yet again opposed these common-sense measures to protect America's children," he said.

Senator Patrick J. Leahy, Democrat of Vermont, who was the only member of the Senate to vote against the law, said he had warned that the law would not withstand a constitutional challenge. "Technology has continued to produce better solutions than this law offers," he said.

Ann Beeson, associate legal director of the A.C.L.U., who argued the case at the court, said she was confident that the law would eventually be struck down. "We urge John Ashcroft to stop wasting taxpayer dollars in defending this unconstitutional law," she said.

There was some evidence that the outcome of the case shifted during the nearly four months that the court had it under consideration. It is likely that Justice Breyer initially had the assignment but lost the case to Justice Kennedy as the result of a change of heart by another justice, perhaps Justice Thomas. The result left Justice Breyer without a majority opinion from among the dozen cases the court heard in March, while Justice Kennedy had two majority opinions.