Web sites that publish inflammatory information written by other parties cannot be sued for libel, the California Supreme Court ruled Monday.

The ruling in favor of free online expression was a victory for a San Diego woman who was sued by two doctors for posting an allegedly libelous e-mail on two Web sites.

Some of the Internet's biggest names, including Amazon.com, America Online Inc., eBay Inc., Google Inc., Microsoft Corp. and Yahoo Inc., took the defendant's side out of concern that a ruling against her would expose them to liability.

In reversing an appellate court's decision, the state Supreme Court ruled that the Communications Decency Act of 1996 provides broad immunity from defamation lawsuits for people who publish information on the Internet that was gathered from another source.

''The prospect of blanket immunity for those who intentionally redistribute defamatory statements on the Internet has disturbing implications,'' Associate Justice Carol A. Corrigan wrote in the majority opinion. ''Nevertheless... statutory immunity serves to protect online freedom of expression and to encourage self-regulation, as Congress intended.''

Unless Congress revises the existing law, people who claim they were defamed in an Internet posting can only seek damages from the original source of the statement, the court ruled.

The case centers on an opinion piece sent via e-mail to Ilena Rosenthal, a woman's health advocate who runs various message boards and promotes alternative medicine.

The scathing missive, written by Tim Bolen, accused Dr. Terry Polevoy, of Canada, of stalking a Canadian radio producer and included various invectives directed at Polevoy and Dr. Stephen Barrett, of Pennsylvania. The two doctors operated Web sites devoted to exposing health frauds.

After Rosenthal posted the piece to two newsgroups, Polevoy and Barrett sued her, Bolen and others for libel. The lawsuit accuses Rosenthal of republishing the information after being warned it was false and defamatory.

The trial court ruled that Rosenthal's actions were protected, but an appeals court decided she was not shielded from liability as a distributor of the information. The state Supreme Court's ruling reversed that decision.

Legal experts said the ruling follows similar decisions in other states designed to protect free and open access to information.

''Even though the court recognizes that it could have unfortunate consequences, they're saying that Congress controls this area,'' said Carl Tobias, a law professor at the University of Richmond.

Christopher Grell, the lawyer for Polevoy and Barrett, said they have not decided whether to appeal the decision.

''What this decision does is, it basically promotes the distribution of offensive material, which I can't imagine Congress ever intended,'' he said.