The Supreme Court said Tuesday it will decide if public libraries can be forced to install software that blocks sexually explicit Web sites, the latest in Congress' string of attempts to shield children from Internet pornography.

Congress has struggled to find ways to protect children from online smut without infringing on free speech.

The latest challenge involves the third law passed since 1996. The Supreme Court struck down the first and blocked the second from taking effect.

The latest measure, signed by President Clinton in 2000, required public libraries receiving federal technology funds to install the filters on their computers or risk losing aid.

A federal three-judge panel ruled that the Children's Internet Protection Act violates the First Amendment because the mandated filtering programs also block sites on politics, health, science and other non-pornography.

"Given the crudeness of filtering technology, any technology protection measure mandated by CIPA will necessarily block access to a substantial amount of speech whose suppression serves no legitimate government interest," the judges wrote earlier this year.

The Bush administration argued that libraries are not required to have X-rated movies and pornographic magazines and shouldn't have to offer access to porn on library computers.

The judges had recommended less restrictive ways to control Internet use, like requiring parental consent before a minor is allowed to use an unfiltered computer or requiring a parent to be present while a child surfs the Net.

The American Library Association and the American Civil Liberties Union had challenged the law.

Paul M. Smith, the attorney for the library association, said more than 14 million people use libraries for Internet access. The latest restriction "takes a meat ax approach to an area that requires far more sensitive tools," he argued.

Smith said that with filtering software, librarians would have no involvement in blocking decisions.

Texas had asked the Supreme Court to uphold the law.

"Parents should not be afraid to send their children to the library, either because they might be exposed to such materials or because the library's free, filterless computers might attract people with a propensity to victimize children," wrote Texas Attorney General John Cornyn, who was elected to the U.S. Senate last week.

Congress knew that the latest law would be challenged, and directed any appeals to go straight to the Supreme Court after a trial before a three-judge panel.

Solicitor General Theodore Olson said the lower court panel's ruling hurts Congress' effort to ensure that money spent "for educational and other purposes does not facilitate access to the enormous amount of illegal and harmful pornography on the Internet."

Olson said librarians answer more than 7 million questions a week and should not have to be worried about on-line pornography.

The Supreme Court struck down the 1996 Communications Decency Act, which made it a crime to put adult-oriented material online where children can find it.

Earlier this year the court upheld part of the 1998 Child Online Protection Act, which required Web sites to collect a credit-card number or other proof of age before allowing Internet users to view material deemed "harmful to minors."

But justices did not rule on the law's constitutionality, and the government was barred from enforcing it.

The case is United States v. American Library Association, 02-361.