Shares of First Chicago NBD Corp. shot up 6.93% Tuesday to $79.0625 on speculation that it had agreed to be bought by Columbus, Ohio-based Banc One Corp. Officials from the banks were unavailable Tuesday or declined to comment.

Such a combination would create a $226.4 billion asset bank with an especially strong credit card business. Its $50 billion in receivables would surpass that the largest card issuer, Citicorp, which has $45.8 billion.

However, analysts said such a merger would be unlikely. Banc One and First Chicago are about the same size, with $113 billion in assets each. Banc One, which ended Tuesday's trading at $51.50, down $1.625, has said it would not acquire a bank more than one-third its size. Banc One chairman John B. McCoy has said he's never seen a successful merger of equals among banks.

First Chicago, formed as a merger of equals two years ago, would also be too large an acquisition for Banc One, said Lehman Brothers Inc. analyst Michael Plodwick. Mr. Plodwick said in addition that both companies have roughly the same market capitalization-First Chicago $21.8 billion and Banc One $21.2 billion.

Tuesday's activity marks the second time in 12 months that First Chicago has been rumored to be for sale. Earlier this year, San Francisco-based BankAmerica Corp. was reportedly going to buy the Chicago bank. BankAmerica later said such an acquisition was not in its near-term strategy.

More logical buyers for First Chicago, said Mr. Plodwick, would be NationsBank Corp. of Charlotte, N.C., and U.S. Bancorp of Minneapolis, both of which have higher market capitalizations.

Moreover, Mr. Plodwick said he does not believe First Chicago is preparing to sell. "We view First Chicago as a buyer, rather than a seller right now," he said.

Analyst Anthony Davis with SBC Warburg Dillon Read & Co. said the combination of First Chicago's credit card operations with Banc One's credit card subsidiary, First USA, would create a dominant player in that business, but little else about a deal between the two companies would make sense.

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