An unidentified firm has offered to buy Smith's Transfer Corp., of Staunton, Va., the nation's eighth largest trucking company.

R. R. Smith, chairman and chief executive, revealed the takeover proposal yesterday, but provided no details.

Smith said he "was approached by another corporation which desires to acquire Smith's Transfer," but added, "no firm proposal has been received."

Smith is the largest stockholder of Smith's, whose shares are traded on the New York Stock Exchange. The stock has sold this week for about $21 a share.

At that price the company's 2.5 million shares would be worth more than $50 million.

The Staunton truck line is a $250 million a year business that specializes in "less-than-truck-load" general freight hauling.

R. R. Smith -- whose brothers founded the company by hauling cattle to market for Shenandoah Valley farmers -- remains the largest stockholder, owning 220,000 shares, about 8.7 percent of the 2.5 million shares outstanding.

Besides Smith, 65, other major shareholders include R. P. Harrison of Waynesboro, Va., a former Smith's executive who owns 5.3 percent, and Emini Fund Inc. of Valley Forge, Pa., which holds 5.3 percent of the shares.

Smith's has expanded aggressively by buying other truck lines, adding three lines in the past two years. Earlier this month Smith's announced it had agreed to buy Garrison Motor Freight, an Arkansas trucker, for an undisclosed price.

Smith's also recently took over M R & R Trucking, a Florida line and Reliable Truck Line, an Alabama firm, and purchased several routes from Akers Motor Freight.