McLean Trucking Co., the nation's fifth-largest freight concern, announced yesterday that it has signed a definitive agreement to be acquired by Meridian Express Co., a privately held transportation holding company based in Dallas.

McLean Chairman and Chief Executive Amory Mellen Jr. and Meridian President Elbert Mitchell said the agreement has been signed by the two companies and unanimously approved by both boards of directors.

The agreement calls for McLean shareholders to receive $18 in cash for each share of stock they hold. With more than 5.6 million shares outstanding, the transaction is worth more than $101.3 million.

The combination is subject to the approval of McLean shareholders, the Interstate Commerce Commission and other regulatory authorities.

McLean, based in Winston-Salem, N.C., operates trucking services in 43 states. The company said it would continue to operate under its present management as a wholly owned unit of Meridian.

According to data submitted to the ICC, McLean lost almost $1 million on total revenues of almost $600 million in the 12 months ending Sept. 30, 1981. It lost a little more than $1 million on revenues of $591 million the prior 12-month period. McLean reported a loss of $3.9 million in the fourth quarter of 1981.

Meridian owns subsidiaries that operate two major trucking companies in the West: Merchants Fast Motor Lines Inc. of Abilene, Tex., and Delta Lines Inc. of Oakland, Calif. In its report to the ICC, Merchants said it had profits of $6.5 million on revenues of $104.4 million in the 12 months ended Sept. 30, 1981. Merchants operates primarily from Texas into Arizona, New Mexico and Colorado.

Delta, which operates from California into Oregon, Washington, Nevada and Arizona, reported a loss of $1.7 million in the same 12-month period on revenues of $146.7 million.

James D. Parker, a securities analyst with Robinson-Humphrey in Atlanta, called the proposed merger "a very positive development for McLean."

For a variety of reasons, the company has been at the break-even or small-loss level, but has been in sound financial condition, he indicated. Parker said the company expanded too rapidly in 1979, lost some of its truck-load traffic to other companies as entry to the industry expanded, and has suffered a loss of freight volume as a result of the recession.

"What it needs is a turnaround in volume; I think we'll see that with an upturn in the economy.

"This is certainly a favorable development for shareholders," he added. McLean's stock, which has traded between $6.50 and $11.75 in the past year, closed Tuesday at 8 7/8 in composite trading on the New York Stock Exchange. McLean had asked that trading be suspended Wednesday pending an announcement following its late-in-the-day board of directors meeting. Yesterday, the stock was the biggest percentage gainer, closing at $14.

McLean said it expects to file proxy materials with the Securities and Exchange Commission as soon as practical so that shareholders can vote on the transaction at its annual meeting in April.