PALM BEACH GARDENS, Fla. (MarketWatch) -- The Federal Deposit Insurance Corp. is gearing up for the prospect of a large bank failure. So double-check that all your deposits, including interest, are well within FDIC insurance limits.

The agency seeks comment by April 14 on a proposed rule designed to help it make a quick insurance determination amid an increasingly complex quagmire of FDIC rules and tough-to-figure-out bank accounts.

One section would place a provisional hold on a fraction - say, 10% or so -- of certain account balances at some 159 of the nation's largest banks. The hold could affect some accounts with balances under $100,000.

If you have uninsured deposits at a bank, should you worry? Possibly. Depositors without FDIC coverage lost money in at least two recent failures -- NetBank, Alpharetta, Ga., and Miami Valley Bank, Lakeview, Ohio.

Of $109 million in uninsured deposits at NetBank, nearly 30% has not yet been reimbursed. Of $14 million in uninsured funds at Miami Valley, only 5.9% of uninsured funds, so far, has been reimbursed. All deposits in the most recent failure -- Douglass National Bank, Kansas City, Mo. -- have been reimbursed.

Fortunately, FDIC insurance limits have increased on certain accounts in recent years. Certain retirement accounts, for example, now are insured to $250,000, up from $100,000 per person.

But the tide on FDIC reimbursement of uninsured depositors may have changed in 1991 for the worse. Congress sharply curtailed the FDIC's discretion to extend protection beyond insured deposits. Now the FDIC must enter into the "least costly" transaction when dealing with a troubled bank. So the FDIC won't reimburse uninsured depositors if it means increasing the loss to the deposit insurance fund.

"As a result, uninsured depositors are protected only if a bank acquiring the failed bank will pay more for all of the deposits than it would for insured deposits only," said FDIC spokesman David Barr.

By contrast: "Prior to December 1991, the only time that the insurance limit was imposed was when we could not find a buyer for a troubled bank and had to issue checks to depositors for their uninsured funds. The overwhelming majority of the time, we were successful in finding a buyer."

Big and bigger

Weren't you reassured about our deposit-insurance system with the bailouts of three colossal banks in the past -- Continental Illinois, First Republic and Bank of New England? Each large bank approached an eye-popping $40 billion in assets. Today, however, a bank of that size would not rank in the top 40, FDIC chairman Sheila Bair warned in a speech last year.

FDIC data indicate that as of Sept. 30, there were 65 institutions with assets of $18.5 billion on its list of "problem" institutions. Barr would not elaborate on their sizes. Nor will the FDIC name the institutions.

Institutional Risk Analytics, Torrance, Calif., based on FDIC data from that same date, puts Bank of America Corp. BAC, -1.32% , Citigroup Inc. C, -2.12% , J.P. Morgan Chase & Co. JPM, -0.84% , Wachovia Corp. WB, -3.74% and HSBC Holdings PLC HBC, +0.48% , as the riskiest big banks. More recently, Managing Director Chris Whalen cited J.P. Morgan, Citigroup and Bank of America as his chief concerns due to their heavier trading activity.

He stresses that there is a 45-day lag time from the close of a quarterly period and the publication of FDIC data. Bank conditions can deteriorate very quickly. Fourth quarter 2007 FDIC data won't be released until late February.

Nevertheless, Whalen doubts that even uninsured depositors at those banks need worry.

"Uncle Sam is not going to let any of them fail," he declared. Some investors, though, could take "haircuts."

Be on guard

So what should you do if you deposit large amounts at a bank?