Here's the latest poop on what a big slug of institutional buying power -- over 700 portfolios with combined assets of about $55 billion -- is presently doing in the stock market.

For starters, the institutional biggies are reassessing the energy stocks. All-over-the-lot buying has been replaced by selectivity. There's also a clear preference for "selected" defense stocks, a brisk amount of selling in the drugs and an unmistakable desire to continue to maintain above-average cash reserves despite the market's continuing robust behavior.

The three stocks attracting the biggest buying on the Big Board: Raytheon, W. R. Grace and Kodak. And the three under the greatest selling pressure: Exxon, Squibb Corp. and Schlumberger.

These trends -- which reflect the market activity of such institutional investors as state funds, banks, insurance companies and investment counselors -- emerge in a computer analysis of those 700-plus portfolios just completed by Indata Services Co. of Georgetown, Conn. Indata, a statistical services company which, among other things, monitors and measures institutional market performances weekly, provides probably the most up-to-the-moment look at what big investors are doing. Unlike some other services whose institutional reports are several months old, the Indata findings are up to date. Our charts show August's results.

Michael Haskett, managing partner of Indata (whose services are marketed through the Big Board member firm of Bruan. Gordon & Co.), tells me the average portfolio is sitting with about a 12 1/2 percent cash position. Since that's near the high end of the range (8 to 15 percent) allotted for cash reserves, it indicates to Haskett that the average institution continues to worry about unsettling economic conditions, plus the direction of inflation and interest rates. It also points up a lot of fire power available on the sidelines to continue to fuel the current rally.

In an accompanying chart, Indata pinpoints the largest institutional purchases -- and sales -- in August. Covered are Big Board, Amex and over-the-counter issues.

Among the top Big Board purchases, two defense issues -- Raytheon and General Dynamics -- are among the top six. Only one oil stock -- Mobil -- makes the top 10. On the other hand, four of the 10 largest sales on the Big Board -- Exxon. Schlumberger, Zapata Corp, and Amerada Hess -- are energy-related. Squibb, Smithkline and Eli Lilly dominated the sales of drug stocks.

Technology stands out among the largest Amex purchases, led by Paradyne and Wang Laboratories. On the sell side: Imperial Oil A and Glatfelter Ph.

Over the counter, Transport Life Insurance was the No. 1 purchased stock in August; on the sell side (somewhat misleading) is ERC Corp., which was acquired by Getty, the shares sold were those tendered. After ERC comes Boston Digital.

Well, now you know what the biggers are doing. Before rushing out and doing the same, a word of warning: Historically, about 80 percent of the institutions under-perform the Standard & Poor's index of 500 stocks, one of the market's best regarded indexes. Haskett tells me the Indata portfolios do substantially better, but he declined to be specific. If nothing else, since the institutions move markets, it's always worth knowing how the big bucks are flowing. CAPTION: Chart, Tracking institutional biggies -- $55 billion, Copyright (c) 1980, The Chicago Tribune