Tribune Co., which is wrapping up a five-month auction, has a new suitor: Chicago real estate billionaire Sam Zell.

Zell -- who recently sold his Equity Office Properties Trust to Blackstone Group for $39 billion, making at least $1.8 billion in the deal -- has become a serious bidder for Tribune's 11 newspapers, 25 broadcast stations and the Chicago Cubs, said a source familiar with the offer who spoke on condition of anonymity because the bid is not public.

The Tribune board is concluding an auction spurred by the Chandler family, the company's largest shareholder, with 20 percent of Tribune stock and three seats on the board.

The Chandlers are upset that Tribune stock has lost nearly half its value over the past three years and have said they believed the company would be worth as much as $46 per share sold or broken up. Tribune stock closed up 6 cents per share Friday, at $30.70.

Before Zell's offer, the auction brought only two serious bids -- one from the Chandlers and one from Southern California billionaires Eli Broad and Ronald Burkle. Tribune's board is enamored of neither bid and has been moving toward a "self-help" plan that would split off the company's television stations into a new company and borrow heavily to pay investors a dividend.

Now, another billionaire has joined the bidding.

Tribune management said it will conclude the auction and pick a plan by the end of next month. The board had largely settled on some form of a self-help plan until Zell entered the auction, the source said.

Tribune is valued at $7.3 billion, and it is unclear how Zell would finance the deal, though it is likely that he -- like the other bidders -- would borrow heavily and possibly bring in private-equity partners.

Zell has never owned a media company but became the nation's largest owner of office buildings, starting in the business as an apartment manager while an undergraduate at the University of Michigan.

He earned the nickname "Grave Dancer" for his ability to spot assets mistakenly passed over for dead by others. He is a native Chicagoan and a large public figure in Chicago -- as was the Tribune's longtime owner and editor, Col. Robert McCormick.

The Chandler plan would split off Tribune's television stations into a separate company, take the papers private with the assistance of private equity and pay the family for about half of its $1.4 billion stake in Tribune. The Tribune board frowns on this offer, it has said, because it believes that the offer favors the Chandler holdings over those of other shareholders.

Broad and Burkle have offered a recapitalization plan that would borrow more than $10 billion, sell off non-core assets such as the Cubs and give the billionaires six seats on an expanded 16-seat board. The Tribune board dislikes this plan because it would give up too much control to the billionaires.