The full impact of Maryland National Bank's decision to transfer its bank card subsidiary to Delaware won't be known for some time yet, but the initial reaction of many owners of the bank's MasterCard is that they will stop using it.

At the heart of their protest is Maryland National's imposition of an annual $18 fee for the privilege of using its MasterCard and Visa card.

The more vocal protesters have accused Maryland National of using a ploy to maintain customer loyalty by reporting only minor losses from its account base.

Despite vowing never to use their cards again, some of the same consumers may decide to keep their accounts active at Delaware subsidiaries if interest rates in Maryland increase substantially.

The banking industry in Maryland has lobbied heavily for elimination of usury limits and for the right to charge fees for credit cards, maintaining that, without this relief, banks will continue to lose money on their credit card accounts.

Unsuccessful in their bid for the changes, major banks in Maryland are preparing to move their credit operations to Delaware where there is no usury limit and where fees are permitted.

Maryland's General Assembly is expected to approve an interest rate ceiling of 24 percent on consumer credit transactions, but there remains strong resistance to the fee proposal.

Maryland National, which is the first Maryland bank to open a subsidiary in Delaware, charges a maximum 18.9 percent annually on credit card transactions and an annual fee of $18. Under existing Maryland law, bank card customers pay 18 percent on the first $700 of credit transactions and 12 percent on remaining balances.

Ironically, using a bank card issued by a Maryland bank with a Delaware operation could cost less than making purchases with one issued by a bank that elects to stay in Maryland.

The average balance on bank card accounts is estimated at $560. Under existing Maryland law, a customer would pay $100.80 a year in interest on that balance.

At 18.9 percent currently charged by Maryland National's Delaware subsidiary, a MasterCard or Visa customer would pay interest of $105.84 a year.

But at 24 percent in Maryland, interest on the same balance would be $134.40 a year.

Although other banks that plan to move their credit card operations to Delaware haven't said what fees and interest they will charge, it's a safe assumption that they will try to be competitive with Maryland National.

And when an $18 fee is added to interest imposed by Maryland National, for example, the total is less than the $134.40 that would be charged at the 24 percent level.

None of those choices may prove satisfactory to some consumers, however. Some may elect to pay off their credit balances within 30 days and avoid interest charges.

Others may reject the concept of a fee and give up their cards as a matter of principle.

As a result of the move to Delaware, Maryland National lost MasterCard accounts in the first two weeks, but less than 10 percent, according to the institution's president.

The customer loss initially was "far fewer than we anticipated," Maryland National President Alan P. Hoblitzell Jr. said last month, referring to the number of persons who either had returned their cards or had informed the bank of their intentions to do so.

But some customers, apparently having misinterpreted Hoblitzell's statement, insist he couldn't possibly know how many persons had decided to discontinue using their cards.

After all, they point out, Maryland National had advised its customers that if they used their cards after March 15 they automatically would be charged the annual $18 fee. How can Maryland National tell how many customers plan to use ther cards if there is no requirement to return them? some irate customers have asked.

Indeed, it is impossible for Maryland National to tell at this stage how many of its MasterCard customers plan eventually to relinquish their cards or discard them in protest. Officials of the bank agree.

"It will probably be June or July before we can come up with some attributable numbers," says Maryland National spokesman Dan Finney.