An unnamed company has offered to buy controlling interest in Drug Fair and to merge with the chain, the area's second-largest.

The opening in trading of Drug Fair stock was delayed yesterday pending an announcement by the company that an offer had been made at an undisclosed price to acquire stock owned by the families of Milton Elsberg, president, and Myron D. Gerber, chairman. Together the families own about 47 percent of Drug Fair's oustanding common stock.

Drug Fair officials refused to say from which company the offer came, but said that the proposal is under "active consideration."

Although several rumors flew around town about the name of the proposed purchaser, the most frequently named possibility was the Jack Eckerd Drug Co. of Clearwater, Fla. It has been rumored for almost a year that the Florida company is interested in expanding its Carolina division north into this area.

Neither Drug Fair officials nor Eckard officials directly denied the rumor, but instead simply refused to comment on it.

Making the announcement, Drug Fair officials said that if and when the merger is consummated, other shareholders would receive the same cash consideration for their shares as the Elsberg and Gerber families would get. After a brief delay, trading in Drug Fair stock resumed about 11:30 a.m.

Drug Fair stock, which is traded on the American Stock Exchange, closed at 13 3/8, up 2 1/2 and its highest level in more than a year. The highest level the stock reached in 1980 was 11.

The 42-year-old drug chain, founded by Elsberg and Myron Gerber's father, has had its ups and downs over the past year or so. In fiscal 1980, the company had what Elsberg described as a "very strong rebound" in its final quarter following a loss during the first half.

The loss had been attributable to the durg chain's Active Casuals Division, which operated a chain of Wrangler Ranch retail clothing stores for two years. Drug Fair discontinued the clothing stores and most of its Soup'r Scoops sandwich and ice cream stores in March 1980.

Those operations had been a disappointing part of an overall campaign to revamp the chain's image. The campaign also included remodeling old stores.

Earnings for this year have been $299,000 in the first quarter, $268,880 in the second quarter and $178,000 in the most recently reported quarter ended Oct. 25. Year-end earnings have not been reported yet.