Acting on a challenge by a New York City firm, a federal court declared that a California law violated the First Amendment of the Constitution by requiring Web sites to obtain brokerage licenses for advertising real estate but exempting newspapers that publish similar information.

The Web site ForSaleByOwner.com, which is based near Madison Square Park and acts as an Internet classified page for real-estate listings, challenged the law after the California Department of Real Estate began enforcing it last spring. Many states have similar laws but few actively enforce them, said a spokesman for the Institute for Justice, Steve Simpson. The Washington, D.C.-based organization represented the Web site.

“This ruling sets a precedent that could prevent discrimination of Web sites by mainstream media across the country,” he said.

Under the California law, Web sites such asForSaleByOwner.com, which allows individuals to buy and sell homes without real-estate brokers, need to get a broker’s license.

“We are not brokers, and we don’t pretend to be,” said the company’s chief operating officer, Colby Sambrotto. “The whole point of our business model is that we are an alternative to brokers, and by getting a broker’s license we would have failed.”

“The fact is, under this law, newspapers that have Web sites exactly like ForSaleByOwner.com are exempted from needing a license, but any Web sites not attached to newspapers do, which is discriminatory,” Mr. Simpson said.

Officials of the California Department of Real Estate, Judge Morrison England Jr. of the District Court in Sacramento wrote in the decision, “vaguely attempt to paint newspapers as geographically situated and relatively more stable than Internet companies.” But the ruling said: “They have not established why this should require Web sites like ForSaleByOwner.com’s to obtain a California broker’s license … when online services doing exactly the same thing are not subject to any licensing requirement so long as they are operated by a newspaper.” In sum, the court saw “no justification whatsoever for any distinction between the two mediums.

Referring to the date when the newspaper exemption was passed, the court added, “even if a distinction was warranted in 1959, that does not mean that the same rationale for exempting the newspapers remains viable in 2004, given the vast advances in technology that have occurred in the meantime.”

The defendants also argued that the California law violated the First Amendment because it required a license to publish information. “The connection to the First Amendment is not immediately apparent,” Mr. Simpson said, “but requiring a license for a broker and a client, or an attorney and a client, is one thing, but it becomes a freedom-of-information issue when, say, newspapers are required to get licenses to publish.”