and ICE have been clearing more over-the-counter contracts, but their main futures business generates higher margins.

"What the exchanges care about is volume," said Craig Pirrong, a professor at University of Houston. "If one or two of these big guys move to the over-the-counter markets or cuts down trading substantially, it could be a big deal."

At a recent roundtable convened by the CFTC, players such as bond fund giant Pimco, mutual fund company Vanguard and the California Public Employees' Retirement System made clear that their main concern was that their money would be as protected in the futures markets as it is in the over-the-counter markets.

CME has been working to hasten the return of, and protect, all customer funds, a spokeswoman said.

Former MF Global clients Coca Cola Co, Hormel, General Mills , Anheuser Busch, Tesoro and Marathon Petroleum declined to comment for this story or did not return calls for comment.

CME DIRECTORS

One measure of how MF Global's bankruptcy caught even savvy investors by surprise is the five CME Group directors who filed claims.

Two of them -- former vice chairman Charles Carey and director Joseph Niciforo -- are suing the broker. Jack Sandner, a former CME chairman, and director Robert Corvino also had small claims, while director David Wescott had $2.1 million in an MF Global account. All declined to comment or did not respond to requests for comment.

Glenn Pankau, whose brother Ron is on the CME board, had banked some $650,000 at his MF Global account, much more than the $50,000 he needed to secure his personal trades.

"I was always told it was the safest place my money could be," Pankau said.

On the Friday before the bankruptcy, Glenn Pankau had a conversation with Wescott, a CME director and MF Global officer. Wescott told Pankau he should take his money out of MF Global if he was uncomfortable.

Pankau tried, but the check bounced. To date Pankau has retrieved only 66 percent of his money from MF Global.

Wescott has received none.

Now, Pankau keeps only enough money at his new broker to back his trades. "I am not going to be victimized again," he said. (Reporting by Ann Saphir and Tom Polansek; Editing by David Gregorio)