The largest bank holding company in the Southeast has taken its first step into the Washington area by agreeing to buy a small thrift institution in Baltimore for approximately $27.2 million in cash and stock warrants.

NCNB Corp. of Charlotte, N.C., has agreed to buy CentraBank of Baltimore pending approval of shareholders and state and federal regulators, the two companies announced yesterday. The merger cannot be completed until mid-1987.

The acquisition marks NCNB's first move to cross into Maryland, Virginia and D.C. under regional interstate banking laws newly enacted by the three jurisdictions. NCNB is a fast-growing bank holding company that has emerged in recent years as an industry giant, ranking 22nd in the nation at the end of last year.

The merger of NCNB and CentraBank -- one of two state-chartered savings banks in Maryland -- was made possible under an amendment to Maryland's interstate law. The amendment, which Maryland Gov. Harry Hughes signed Tuesday and CentraBank says it lobbied to get, gives state savings banks the same right that banks have under Maryland's regional banking law, including the right to be purchased by banks from other states within the region.

Savings banks are thrift institutions that are owned by depositors. Without the amendment, the merger would be illegal.

NCNB, which has 600 offices in North Carolina, South Carolina, Florida and Georgia, cannot use the Baltimore bank as a base for branching into D.C. and Virginia, but can enter those states by buying additional banks. NCNB had assets of $23 billion at the end of March.

Centrabank has assets of more than $225 million. It has eight branches in the Baltimore area.

NCNB said it will pay $8.7 million cash and issue 350,000 warrants to Centrabank depositors of record as of Dec. 31, 1985. NCNB then will convert the savings bank to a full-service, state-chartered bank and change the CentraBank name to NCNB of Maryland, said NCNB Vice President James M. Berry.

For five years after the merger, each NCNB warrant can be exchanged for one share of NCNB common stock at a predetermined price. At the end of that period, unused warrants may be exchanged for $10, NCNB officials said.

At NCNB's closing price of $52.75 on the New York Stock Exchange yesterday, the warrants would be worth $18.7 million. The actual value of the warrants will be the average price of NCNB stock in the six days before completion of the merger, which is expected on July 1, 1987, the soonest legally possible

Maryland's law now allows interstate mergers of banks with headquarters in Delaware, Virginia, West Virginia and Florida. Next year, nine states will be added to the list: Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, North Carolina, South Carolina and Tennessee.

NCNB has said it is very interested in the Baltimore/Washington area, which it says has a combined deposit base that ranks 6th in the nation and is among the fastest growing. NCNB officials said that purchases in Northern Virginia are high on their list.

CentraBank President and Chief Executive H. Lee Boatwright said the thrift's directors will become directors of the new Maryland bank and that all its 130 employes will work for the new entity. He said that none of the directors owns CentraBank stock.

Maryland officials said that the state's other thrift institutions -- stock equity thrifts owned by shareholders, not depositors -- are not included in the interstate law because Maryland wants to sell those thrifts, many of which are ailing, to the highest out-of-state bidder, regardless of where they are based.

In an unrelated merger, Chemical Bank, the nation's sixth-largest bank holding company, said it would buy Horizon Bancorp., New Jersey's fifth-largest bank holding company, for about $465 million.