NCNB Corp. of North Carolina, the largest bank-holding company in the Southeast, has agreed to purchase Prince William Bank for $4 million, bank officials said yesterday.

The agreement is the second this month in the Washington area for NCNB, which has its headquarters in Charlotte.

The company announced a merger agreement valued at $92.5 million with Ameribanc Investors Group, a D.C. holding company with savings and loan offices in Virginia, on July 7. NCNB officials say they plan to use the Prince William Bank charter as a tool for converting the Ameribanc offices into banks.

Prince William Bank, which is based in Dumfries, Va., has assets of $20 million, compared with NCNB's $23.9 billion in assets. One advantage of the purchase of Prince William Bank is that it gives NCNB a presence in the affluent Washington suburbs, bank officials said.

In an interview with The Washington Post in December, NCNB Chairman and Chief Executive Hugh McColl said the company soon would be seeking acquisitions in Virginia, Maryland and, perhaps, the District.

NCNB, the 17th-largest bank-holding company in the nation, operates nearly 600 banking offices in North Carolina, South Carolina, Florida and Georgia. It is awaiting state and federal approval of its purchase of CentraBank of Baltimore, which it agreed to buy for about $27.2 million in cash and stock warrants in May.

The purchase of Prince William Bank gives NCNB a vehicle to convert Ameribank's First American Savings and Loan Association of Annandale, which has 30 offices in Virginia, into retail banks, an NCNB spokesman said.

Under the reciprocal interstate banking law in Virginia, NCNB needed to purchase an institution that had operated with a banking charter for at least five years to enter retail banking in the state, the spokesman said.

"Yes, in fact, the Prince William bank qualifies and is a vehicle into which we will roll Ameribank," said NCNB spokesman Dick Stilley.

"In Florida, we've converted and consolidated into one state-wide bank organization. That's a blueprint of how we work generally," he said.

An official of the Federal Reserve Board, which regulates bank-holding companies, declined to comment on whether NCNB's purchase of the Prince William Bank will enable it to convert Ameribank's operations. The purchase of Prince William Bank is subject to approval by stockholders and state and federal regulatory agencies.

The $21-per-share price for Prince William's 190,316 outstanding shares, which are privately held, represents a 300 percent increase in the value of the stock during the past two years, said Roger P. Pilcher, president and chief executive officer of the bank.

The 15-year-old bank has a branch in Dumfries and one in Woodbridge, in addition to its Dumfries headquarters. Pilcher and the bank's 27 employes will remain with the bank, Stilley said. The bank will likely be called NCNB of Virginia.