Carolina Financial advanced 8.9% to $42.03. United Bankshares was off 3.9% to $37.98.
United Bankshares said it would pay 1.13 of its shares for each share of Carolina Financial.
The boards of both companies have approved the terms. Subject to regulatory clearances and approval by the holders of both companies, United and Carolina expect to close the deal in the second quarter of 2020.
The combined company will have about $25 billion in assets and rank in the top 35 largest banks in the U.S. based on market capitalization.
Carolina Financial Chief Executive Jerry Rexroad will join United Bankshares as a director and executive officer.
United Bankshares, headquartered in Washington, D.C., and Charleston, W.Va., is a financial holding company with about $19.8 billion in assets. It's the parent of United Bank, the largest community bank headquartered in the Washington metro area. United has locations in seven states plus the District of Columbia.
Carolina Financial, Charleston, S.C., has about $3.98 billion in assets. It's the parent of CresCom Bank, operating 62 locations in North Carolina and South Carolina. CresCom Bank also owns and operates Crescent Mortgage Co., which is based in Atlanta.
Sandler O'Neill & Partners was financial adviser and Bowles Rice provided legal counsel to United. Raymond James & Associates provided financial advice and Nelson Mullins Riley & Scarborough was legal counsel to Carolina Financial.