If you're a believer in the classic investment advice of "buy low, sell high," you may be interested in one of 1986's real estate phenomena: the arrival of mass-market "opportunity funds" run by some of the biggest names in American development and finance.

The opportunity fund concept -- outlined at a national investment conference here last week -- is symbolic of American real estate's current contradictory state of health:

On the one hand, single-family-home building, mortgage lending and refinancing are at their highest levels of the decade in many parts of the country. Mortgage interest rates, whether for residential purposes or investment property, are the lowest they've been since the late 1970s.

On the other hand, billions of dollars worth of apartment houses, condos, office buildings, hotels and other forms of real estate are sitting half empty or worse in large stretches of the Sun Belt and beyond.

Encouraged by generous federal tax-shelter benefits, developers went on an overbuilding binge from 1981 to 1984 in many cities, throwing economic caution to the winds. Even in relatively healthy cities such as Boston, Atlanta and Chicago, vacancy rates in commercial office buildings have risen into the double digits. Energy-slump cities such as Denver and Houston have 25 to 30 percent of their office space sitting unrented.

Worse still, the federally insured mortgage lenders across the country who financed these money-losing buildings are now finding themselves as their ultimate owners. Two hundred of the largest savings and loan associations hold an estimated $5.3 billion in foreclosed real estate in their bulging portfolios. The Federal Savings and Loan Insurance Corp. -- the agency that guarantees the safety of deposits at federally chartered S&Ls -- has more than $3 billion worth of properties on hand from failing local lenders. FSLIC's counterpart in the commercial banking arena, the Federal Deposit Insurance Corp., has $250 million to $300 million worth of ailing real estate to dispose of.

Given all this red ink, could there possibly be legitimate, sound opportunities for sharp investors? Could well-financed organizations buy such real estate at bargain-basement prices and turn them around through better management?

Absolutely, according to representatives of some of the country's most prestigious real estate and finance organizations who gathered for a strategy meeting here. Several top firms already have taken the plunge and are raising millions of dollars for distressed-property war chests.

Mutual Benefit Life Insurance Co. has teamed up with Dallas-based Trammell Crow Residential Cos. to buy "turnaround" apartment buildings. According to officials, the two large firms' multimillion-dollar opportunity fund plans to acquire properties "at prices which have been substantially discounted," and then use their management and leasing skills to build up their value .

Individual investors can buy into the new fund for as little as $2,500. Although no cash flow is projected for the initial years of the fund, resales of the turnaround properties later on could provide significant capital gains, the promoters say.

Other large opportunity, or "vulture," funds are being offered or organized by California-based Consolidated Capital Corp. ($10,000 minimum investment) and Angeles Corp., both major sponsors of real estate limited partnerships. Lincoln Property Co., with projects nationwide, is gearing up a fund aimed at large institutional investors.

But does an investment in a turnaround fund make sense for you personally? Some practical guidelines suggested by speakers here:

*Consider any "opportunity fund" to be higher risk than ordinary, publicly registered real estate partnerships such as those used by many Americans for their Individual Retirement Accounts. By definition, such funds are looking for real estate lemons. Not all of them eventually will produce lemonade.

*Don't bother with funds run by firms that lack long track records in managing their own real estate profitably. Asset-enhancement is the name of the turnaround game -- one that has relatively few top national players.

*Give preference to funds that can specify the type and location of their Cinderella properties. The more you know about the intended transformations, the better able you'll be to judge whether Cinderella is for real or just another real estate promoter's fairy tale.