Bank of Montreal, Canada's third-largest bank, reportedly is preparing to make a major move into the United States market by taking over the third-biggest bank in Chicago.

A spokesman for Harris Bankcorp Inc. confirmed in Chicago that bank officials have had "preliminary discussions with a major international bank concerning the acquisition of Harris Bankcorp." For its part, Bank of Montreal said it is aware of "rumors" that it is talking with Harris, but refused to comment.

The statement from Harris emphasized that it has received no takeover proposal and said it is "impossible to tell" whether a transaction will take place.

If the Montreal bank buys Harris, it would be the first important move into the U.S. market by a Canadian bank in recent years. Banks from Europe, Japan and the Far East have bought or are trying to buy important middle-sized U.S. banks.

The Bank of California, with assets just shy of $3.9 billion, is the object of an acquisition try by Japan's Mitsubishi Bank.

Bank of Montreal, with assets of about $52 billion ($64 billion Canadian), is Canada's most aggressive bank, according to Lawrence Fuller, the chief banking analyst for the brokerage firm Drexel Burnham Lambert Inc.

Fuller said that the acquisition of Harris would make sense. The two banks' spheres of influence would define an "economic region that runs from Illinois north to Montreal," he said.

Rumors of the takeover sent Harris stock soaring. The New York Stock Exchange had to delay the opening of the normally lightly traded Harris stock because of a severe imbalance between buy and sell orders. When the stock opened, its price jumped by $4.25 to $51 a share. About 18,300 shares changed hands within minutes after the stock opened. That is close to a full day of trading in Harris stock in normal times.