Giant Food Inc. entered the ranks of elite U.S. corporations last week when the Landover grocery chain was selected by the Standard & Poor's Corp. to become part of its two blue-chip lists, the S&P 500 and the S&P 400.

That distinction caused Giant Food stock to rise sharply in heavy trading Thursday on the American Stock Exchange. In fact, the demand for stock was so heavy at the opening that trading was delayed for 51 minutes while the specialist tried to match buyers and sellers.

When trading finally began at 10:21 a.m., the price of Giant Food A shares was up $2.63, to open at $37.75 on 87,200 shares. The stock also closed the day at $37.75 on a total volume of 250,000 shares. That was a 467 percent increase in the stock's average daily volume. The week ended Friday with the shares closing at $37.63.

Many of the orders for Giant Food shares apparently came from index funds, which try to mirror the performance of the S&P 500 and 400 stock indexes by investing in S&P stocks in the exact same proportion as the indexes. Thus, when the S&P adds a stock to its list, the index funds must buy that stock to match up.

Index funds in the United States now control hundreds of millions of dollars.

For this and other reasons, Giant Food stockholders may see more action in their stock than they are used to. Stocks that are part of the S&P 500 are often bought and sold as part of the computer-assisted strategy called program trading. In program trading, a pension fund may buy or sell several million dollars worth of S&P 500 stocks while taking an opposite position in the futures contracts on the S&P 500 index.

Program trading, as a strategy, often helps drive the market up or down sharply, especially when futures contracts on the S&P contracts expire.

But it is difficult to predict what effect program trading might have on a single stock issue, such as Giant Food, and to what extent it will be noticeable.

S&P officials told David B. Sykes, Giant Food's chief financial officer, that they consider several factors when choosing S&P 500 companies, including market capitalization. That is figured by multiplying a company's outstanding shares by their price.

For Giant Food, with 30 million shares outstanding, the "market cap" amounts to $1.1 billion.

Old-timers in Washington may have a little trouble thinking of Giant Food as a company that is worth more than $1 billion. But at this rate, $2 billion may not be far away.

Corporations are rarely stagnant enterprises -- especially in these days of high costs and high competition.

Most companies seemed possessed of a certain momentum that is either taking their profits steadily higher or pushing them rapidly downhill. And for those that have made the downhill journey, there is the turnaround stage.

Turnarounds take many forms but, most commonly, companies that are in trouble bring in new managers, cut expenses by cutting employes, reorganize company operations, sell unprofitable divisions or acquire firms that are in harmony with the company's primary goals.

The turnaround process is a tough experience for people who lose their jobs or otherwise find their lives disrupted. But American business is a dynamic process in which profits are the ultimate test of success or failure and the turnaround cure often puts a company back on the road to profitability.

A number of companies in the Washington area share the turnaround experience.

One of the most notable is Black & Decker of Towson, Md., a leading manufacturer of power tools and small appliances, whose fortunes slid badly for several years. The company was reorganized and has begun to turn around.

The improvement has been sufficient to convince analyst Franklin L. Morton at Alex. Brown & Sons, Baltimore, to add Black & Decker Corp. to the firm's "emphasis list," a dozen or so stocks that it considers its "most current and timely buy ideas."

Morton said he recommends B&D for several reasons:

First, cost-reduction programs continue with three plants having closed this summer and, by the end of fiscal 1988, about $75 million to $80 million in overhead expenses is to be eliminated.

Second, small increases in prices and volume can have a significant effect on company profits.

Third, new products are expected to contribute to improved sales and Japanese manufacturers are under great pressure to raise prices on competing products.

Fourth, profits seem to be turning up. In the June quarter, the company reported earnings of 15 cents a share versus 5 cents a year earlier.

B&D shares, which have been as low as $14.50 in the past 18 months, bottomed last fall and have been moving steadily upward into the mid-$20s.

Morton thinks the stock has a way to go.

"A market multiple of 17 times our projected 1989 earnings estimate of $2 to $2.50 could result in a price of $34 to $42 within the next 18 to 24 months," Morton said.

The shares closed Friday at $26.13.

For many corporations in this town, profits are heavily dependent on government contracts. And when a company wins a big, new contract and the champagne corks start to pop, so do the stocks of the companies winning those contracts.

QuesTech Inc. of McLean is a good example of that phenomenon. Two QuesTech subsidiaries were awarded an Army contract that the company says has a potential value of $45 million over a three-year period. The contract has a one-year base period and has two one-year options but Vincent L. Salvatori, executive vice president, said the company is confident that it will get the full job.

QuesTech opened the year at about $6 a share and in early April moved to the $8 level. As profits showed signs of improving, and turnaround hopes grew, the stock gradually rose to the $11 to $12 area. Then, the announcement of the new contract helped move the stock to $15 last week on unusually heavy volume. The stock closed Friday at $14.50. QuesTech's new contracts involve signal intelligence and electronic warfare.

C3 Inc. of Herndon, a computer services company, said recently that its subsidiary, Tempest Technologies Inc., expects to file an initial public offering this year. As its name implies, Tempest Technology specializes in adapting computer equipment to the tempest high-security technology that the government and many companies use to protect the secrecy of their information. ... Heilig-Meyers, the Richmond-based furniture store company, continues to grow. Having absorbed the Sterchi chain, Heilig-Meyers has 225 stores and was planning to open six more during the year. Then, last week, Heilig-Meyers announced it was negotiating to buy 22 more stores in its operating area. Heilig-Meyers sales have reached almost $75 million. ... Data Measurement Corp. of Gaithersburg, in a secondary offering, sold 300,000 shares at $12.63. The stock closed the week at $15.

When interest rates soared in April, ERC International of Vienna dropped its plans to sell $25 million in convertible subordinated debentures to the public. Since then, ERC has worked out a deal with Massachusetts Mutual Life Insurance Co. The insurance company will invest $10 million in 10-year convertible subordinated debentures at 7.95 percent interest. The bonds will be convertible to ERC shares at $17 a share.

Jack E. Aalseth, chairman, said the funds would be used to reduce the company's long-term debt and to help fund acquisitions.