MNC Financial Inc., the region's biggest bank company, sold its credit card subsidiary to investors yesterday for $955.3 million, raising more than enough cash to avert an immediate financial crisis but giving up ownership of its most profitable business in the process.

The 45 million shares of the credit card operation, called MBNA Corp., started trading on the New York Stock Exchange for $22.50 a share and closed at $23.25, evidence of demand for the company, the fourth-largest issuer of bank credit cards in the nation. More than 4.1 million shares of MBNA changed hands yesterday, making it one of the most actively traded stocks on the NYSE.

The completion of the sale will allow MNC, the parent of American Security and Maryland National banks, to pay back all debts coming due in the next six months, including $271 million in loans from its two bank subsidiaries that come due Feb. 4. News of the sale boosted MNC's battered stock, which closed yesterday at $3, up 25 cents.

With the debt payments no longer a worry, MNC is free to concentrate on its $6 billion portfolio of real estate loans, which has been hit hard by the deterioration of the region's commercial real estate market. In the first nine months of 1990, MNC lost more than $240 million because of the real estate downturn.

"Now, MNC can get down to some of the more mundane things, like asset quality and earnings," said David Penn, an analyst who follows the company for the Legg Mason and Co. brokerage in Baltimore. "But it's still not going to be easy for them to earn money."

Analysts said they expect MNC to report further loan losses in the fourth quarter and throughout 1991. Results in 1991 could be worse than 1990, analysts said, because profit from the credit card subsidiary will not be available to offset the loan problems.

MNC spokesman Daniel J. Finney said the sale of the credit card operation went "far better and faster than anyone expected." Goldman, Sachs & Co. had planned to market the stock to investors around the nation this week, but response to initial stock presentations last week was so strong that Goldman canceled much of its marketing tour and priced the deal Monday at $1.50 a share higher than the minimum sale price set by MNC.

Analysts said the interest in the company was not surprising. MBNA has always been the most profitable subsidiary of MNC, contributing as much as 30 percent of the bank company's annual profit. In 1990, while MNC struggled under the weight of hundreds of bad real estate loans, MBNA made $129 million.

Although delinquent credit card payments have increased in recent months because of the downturn in the national and local economies, analysts said most investors believe MBNA will continue to be a highly profitable, well-managed company.

The majority of the 45 million shares were bought by institutional investors -- insurance companies, pension funds, banks and brokerage houses, brokers familiar with the deal said.

To help boost interest in the sale, MNC Chairman Alfred J. Lerner and Progressive Co., his Cleveland insurance firm, purchased 15 percent of the offering, or 4.7 million shares. Progressive Co. is awaiting approval from the Federal Reserve to buy another 10 percent.

Finney said MNC will use proceeds from the sale to shore up the credit card subsidiary so that it can stand alone as a separate bank holding company. As part of the deal to sell MBNA to the public, MNC promised to give the new firm $225 million in capital, the financial cushion that bank companies must have on hand to protect against losses.

In addition to paying off the $271 million in loans from its two subsidiary banks, MNC also will use the proceeds to pay off $382 million of long-term notes that MNC owes investors and to pump another $60 million into Maryland National and American Security to shore up their capital levels.

Sources familiar with MNC said the Baltimore-based bank company will have to continue selling units to provide enough capital for the subsidiary banks to meet federal standards. MNC announced yesterday that it had signed an agreement with General Motors Corp.'s mortgage subsidiary for the sale of $300 million of secured loans from American Security Bank. That sale, while not providing funds that can be used to pay off debt, will free up $25 million in capital for the bank because of accounting rules.