Starting next month, Internet service providers with customers in Pennsylvania will be legally responsible for blocking access to child pornography.

The law, with maximum penalties that include prison time for repeat offenders, is believed to be the first of its kind.

But by putting the onus on the state attorney general's office to notify ISPs of what should be blocked, the law is expected to have limited success.

Under the law, signed last month, prosecutors would, after obtaining a court order, give ISPs a list of Web sites and other items to block.

But child pornographers — many of whom operate from overseas — can quickly move to other sites. Child porn sites are generally temporary fixtures that disappear after a few hours anyway, said John Philip Jenkins, a Penn State professor who has researched Internet pornography issues.

"There's probably more out there than anybody knows, but this probably won't be an effective way of doing anything about it," Jenkins said.

The law carries penalties of $5,000 for the first offense and $20,000 for the second. After that, violators are subject to fines of $30,000 and up to seven years in prison.

The law has the blessing of the state chapter of the American Civil Liberties Union. Larry Frankel, the chapter's executive director, said someone whose material is cut off could seek a court hearing.

But ISPs consider the law impractical.

"Once you use my service to get on the Internet, I have no way of controlling where you go and what you see," said Sue Ashdown, director of the American Internet Service Provider Association, an organization of small ISPs.

ISPs serve as conduits and do not actually control content. They have been likened to the Postal Service, which delivers letters without knowing what is inside the envelopes.

The law does not require ISPs to actively monitor their service, only to block specific sites or services when notified.

But more technically astute users can often bypass blocks by using so-called proxy services.

Kevin Harley, a spokesman for the state attorney general's office, said the agency plans to expand its child sexual exploitation unit and will monitor the Internet for sites that traffic in child pornography.

Two years ago, a congressional commission called for law enforcement agencies to develop a list of Web sites, newsgroups and other Internet destinations that contain child pornography.

The commission's own lists indicated that about 100,000 Web sites show simulated or real child pornography.

Three federal laws aimed at restricting pornography — involving minors or otherwise — have all been challenged in the courts.

One was overturned, another is before the Supreme Court and a third is scheduled for a trial in Philadelphia this month. None requires ISPs to do the blocking.

Officials at America Online and the National Conference of State Legislatures said they knew of no other state law like Pennsylvania's.

A South Dakota law merely requires employees of ISPs to report any child pornography to law-enforcement officials, while South Carolina has a law requiring the same of computer-repair technicians.