Google,, AOL and Yahoo are helping defend online peer Craigslist against a lawsuit that would hold the Web site liable for discriminatory housing ads that appeared on its site.

The online companies last week filed an amicus brief in the U.S. district court case in Chicago, saying a ruling against Craigslist could have “extremely negative implications” for the growth of online services. The brief was also signed by eBay Inc., which owns 25 percent of Craigslist, and the Electronic Frontier Foundation, among others. Chicago Lawyers’ Committee for Civil Rights Under Law Inc. v. Craigslist Inc., No. 06C-0657 (N.D. Ill.).