Fired radio host Don Imus has hired a premier First Amendment attorney in an effort to receive the $40 million left on his contract with CBS Radio, Fortune magazine reported on its Web site yesterday.

CBS attorneys, according to Fortune, said Mr. Imus was fired on April 12 for cause and is not owed the rest of the money.

The report said Mr. Imus has hired Martin Garbus, a New York-based attorney with the law firm Davis & Gilbert. Mr. Garbus, the magazine said, successfully represented comedian Lenny Bruce against criminal charges on First Amendment grounds, as well as writer Robert Sam Anson in a lawsuit filed by Walt Disney Co. trying to halt the publication of a book critical of the media giant.

Fortune quoted one person who has read Mr. Imus’s contract as saying a legal showdown may turn on how language in the contract encouraged the radio host to be irreverent and engage in character attacks is interpreted.

The source told the magazine that the language was part of a five-year contract that went into effect in 2006 and paid Mr. Imus nearly $10 million a year. The contracted stipulated Mr. Imus be given a warning before being fired for doing what he made a career out of — making off-color jokes. The source described it as a “dog has one-bite clause.”

CBS Radio is a unit of CBS Corp.