A long legal drive to shield children from sexually explicit material on the Web ended in failure Wednesday when the Supreme Court let a 10-year-old anti-pornography law die quietly.
In striking down the law on free-speech grounds, the justices said parents could protect their children by installing software filters on their computers.
But less than half of parents do so, Bush administration lawyers had argued in an effort to revive the law.
Anti-pornography activists said the court's action, coming a day after former President Bush left office, signaled the government's bid to restrict pornography on the Web had come to an end.
"The timing puts an exclamation point on it. There's very little reason for hope on this issue," said Patrick Trueman, a Virginia lawyer who headed the Justice Department's anti-obscenity unit from 1988 to 1993. "I don't think Congress will try again to protect children from pornography."
The Supreme Court had struck down an even broader law passed in 1996 that restricted "indecency" on the Internet. After that ruling in 1997, Congress tried again with a narrow measure that targeted commercial purveyors of pornography on the Web. It was signed into law by President Clinton late in 1998.
The Child Online Protection Act made it a crime to put sexually explicit material on a website for commercial gain unless the sponsor used some means, such as requiring a credit card, to keep out minors. It never went into effect.
Judges repeatedly cited free-speech grounds and blocked its enforcement.
The Supreme Court in 2004 said the law violated the 1st Amendment because it would crimp the rights of millions of adults.
In a 5-4 decision, the justices sent the case back to a lower court in Philadelphia to decide if software filters were effective in screening out sexually explicit material.
Last year, the U.S. appeals court in Philadelphia struck down the law as unconstitutional, saying the software filters were "equally effective" as a means of protecting children from online pornography.
In October, Bush administration lawyers disputed that claim and appealed to the Supreme Court. If the law were finally struck down, it "would leave millions of children unprotected from the harmful effects of the enormous amount of pornography" on the Internet, they said.
Countering that contention, the American Civil Liberties Union said the law would crimp free speech on the Web for adults and would not shield children, since at least half of the sexually explicit websites were outside the U.S.
The justices considered the appeal in two closed-door meetings in recent weeks. They issued a brief order Wednesday dismissing the case of Mukasey vs. ACLU without comment.
Steven R. Shapiro, the ACLU's legal director, said the court's decision was consistent with its free-speech principles.
"They have said online speech is entitled to the full protection of the 1st Amendment. That means the government can't adopt a broad censorship measure if there are less-restrictive ways to accomplish the same goal," he said.
In this case, he said, it was shown the software filters were more effective in protecting children from pornography.
Though free speech remains the rule on the Internet, there is one major exception. The government vigorously prosecutes those who trade online or store on a computer sexually explicit material that portrays children. The Supreme Court has made clear that child pornography is not protected as free speech under the 1st Amendment.
When the computer pornography issue came before the high court five years ago, it split the justices but not entirely along the usual ideological lines. Voting to block the law on free-speech grounds were Justices John Paul Stevens, Anthony M. Kennedy, David H. Souter, Clarence Thomas and Ruth Bader Ginsburg.
Also Wednesday, the justices ruled unanimously that victims of sex discrimination in schools or colleges may sue for a violation of their constitutional rights as well as under Title IX of the education code.
Title IX, a 1972 measure, requires schools to offer equal opportunities to women and girls, and it is credited with launching a revolution in women's sports. But it does not allow lawsuits against individuals, such as a principal or teacher, and it does not extend to military service academies.
Some lower courts had said that because Congress passed a specific law to combat sex bias in schools, it meant to limit constitutional suits against schools and school officials.
The Supreme Court disagreed Wednesday and said Congress' move to add one legal remedy against sex bias in schools did not mean it meant to take away another. The decision in Fitzgerald vs. Barnstable Schools revives a suit by parents who said their 5-year-old daughter was sexually harassed for weeks on a school bus by an older boy.
The court did not decide the case but said that the parents could try to show school officials had failed to act on their complaint.