CHARLOTTE, N.C. - A year after making a successful $34.2 billion move into credit cards, Bank of America Corp. found yet another multibillion-dollar opportunity to grab more customers.

The Charlotte bank said yesterday it would purchase LaSalle Bank Corp. from ABN Amro North America Holding Co. for $21 billion in cash.

The purchase fills a big hole in Bank of America's nationwide branch network by making it Chicago's largest bank. Bank of America is a small player in the Philadelphia area, where it has $6.4 billion in deposits, or about 2.5 percent of the market, according to the latest federal figures.

Yesterday's deal was initially disclosed by ABN Amro as part of a broader announcement: The Dutch bank said it agreed to sell itself to Barclays Bank for nearly $91.2 billion.

The purchase of LaSalle raises questions about Bank of America, which is up against a federal cap that bars it from making acquisitions that would give it more than 10 percent of all U.S. deposits. The bank, the nation's second-largest after Citigroup, recently controlled just over 9 percent.

"I think there is a huge opportunity here, but the near-term costs are what people initially see," said Jefferson Harralson, an equity analyst with Keefe, Bruyette & Woods Inc. in Atlanta. "Long term, it's a great strategic move for them."

The net cost to Bank of America will be $16 billion after it gets back $5 billion in excess capital.

Bank of America said it expects the deal to provide $800 million in after-tax cost savings and to immediately enhance its earnings per share. Restructuring costs also are expected to be around $800 million, the bank said.

Investors and Wall Street offered mixed reactions, sending shares of Bank of America down 53 cents, or 1 percent, to close at $50.51 on the New York Stock Exchange.

"We like this deal, particularly as this strengthens BofA in the third-largest deposit market in the U.S.," analysts at Friedman, Billings, Ramsey & Co. said.

For the last several months, Bank of America's chairman and chief executive, Ken Lewis, has expressed his bank's interest in the Chicago market, particularly the strength of LaSalle, in speeches and conference presentations.

LaSalle, based in Chicago, is a top-20 U.S. bank holding company, with $113 billion total assets. Together the banks would surpass the current Chicago market leader, JPMorgan Chase & Co.

LaSalle also has branches in Michigan and Indiana.

In the last four years, Bank of America has increased its retail presence in Chicago from a single financial center to 56 locations. Once combined with LaSalle's 141 Chicago area offices, Bank of America will have more than 14 percent of the deposit market share in Chicago.

Analysts also are questioning how Bank of America will get around the federal deposit-cap law.

In 2004, the bank acquired FleetBoston Financial. A year ago, it added millions of names to its ledger through its purchase of the credit-card issuer MBNA Corp. of Wilmington.

Currently, the bank has pending purchases of the wealth-management company U.S. Trust Corp. and a stake in the student lender SLM Corp., known as Sallie Mae.

Last month, when the Federal Reserve Board approved Bank of America's plan to buy U.S. Trust, it said the buyer held $612 billion in deposits, or 9.1 percent of the U.S. total before the purchase. With LaSalle's approximately $57 billion deposits and U.S. Trust's $9.4 billion deposits, the bank would appear to be slightly over the 10 percent threshold.

Bank officials said they were confident they could meet the requirements.