There once was a time when investing in office high-rises, apartment complexes and shopping centers, like polo, was a game for the very wealthy. Small investors interested in real estate had to be content with six-flats, 12-flats and little else.

But the last 10 years there's been a dramatic change as thousands of Americans have pooled modest sums -- as little as $5,000 each -- to purchase major properties nationwide. The vehicle for this turnabout is the real estate partnership.

An individual or firm that serves as "general partner" buys buildings and operates them, gathering "limited partners" who provide the equity -- the down payment, in simple terms.

Just how much the real estate partnership or "syndication" business has grown is exemplified by Balcor Co., a Skokie, Ill.-based firm that didn't even exist a decade ago. Its limited partners today number about 16,000. In 1980 it invested about $120 million in real estate, and this year it may put out $160 million. It oversees a real estate portfolio valued at more than $1 billion.

Samuel Chase Jr., president of the 3,000-member Real Estate Securities and Syndication Institute, and affiliate of the Chicago-based National Association of Realtors, believes that syndications by 1990 will be a financing source for about $300 billion in development and investment. Small investors banding together in syndications and institutions one day will be the only sources of equity for big real estate deals, Chase predicts.

There are two basic types of real estate partnerships: public and private. Public syndications by definition have more than 35 limited partners. Their offerings must be registered with the Securities and Exchange Commission or, if all the properties are in one state, with the appropriate state authority. Private partnerships have 35 or fewer limited partners and need not be registered.

Public offerings can be purchased through big securities brokerage firms. Private offerings may be a little more difficult to locate. Some can be found through the securities firms. In addition, local real estate firms such as B&B Investment Co. of Chicago, Ill., the McLennan Co. of Park Ridge, Ill., and Inland Real Estate Corp., of Oak Brook, Ill., form smaller investment pools and seek investors directly.

Real estate brokers, lawyers, and accountants sometimes also can recommend private deals.

Public partnerships often offer limited partners admittance with investments as low as $5,000 and, in some cases, even less. For private partnerships, the cash required usually is greater -- say, in the $20,000 to $80,000 range -- although the investment may be made over several years by the limited partners. The holding period of the property, during which time the investment is relatively illiquid, usually is in the range of 40 to seven years for both types of partnerships.

Public syndications, because they raise more capital, may buy different types of properties in different geographical areas. This results in spreading risk. And because offerings undergo SEC scrutiny, the chances of linking up with a shady general partner are reduced.

A private partnership may buy only one property, putting its eggs in one basket. And the general partner doesn't have to "stand inspection" by the SEC.

But supporters of private offering say that, because various brokerage, legal administrative and management fees tend to be less than those in public partnership, more of the dollars invested find their way into brick and mortar, thus enhancing the possibility of high yields.

Most real estate partnerships offer their investors three things: tax shelter, cash flow from rents and other operating revenues and appreciation of the property.

Chase of RESSI says appreciation now provides a major source of profit to investors, a sharp change from the early and mid-1970s. In those days, he said, cash flow was the important element, usually representing an annual return on investment of roughly 8 percent to 11 percent. But because of overbuilding a property's appreciation may have risen only 5 percent to 10 percent a year.

Now, because of high interest rates and high sales prices of prime properties, cash flow typically runs at only 3 percent to 6 percent return, according to Chase. But top-quality real estate in some cases has seen annual rises in value of 15 percent to 20 percent.

"One of the things that we're rather proud of is that we never have had an investor who has lost money," said Donald Fortunato, vice chairman of Balcor. A balcor syndication providing anything less than an average annual yield to the limited partners of 25 percent -- counting tax benefits, cash flow, and appreciation -- would not be considered particularly successful, he said.

Robert McLennan Jr. of the McLennan Co. says his father remembers deals in the early 1950s "that he knew were going to make money and he couldn't get investors. Real estate speculation was thought of as slightly more respectable than a crap game."

But in 1977-78, "it was absolutely scary," McLennan said. "People would just hand you a check. When you have that much 'dumb' money going into any investment medium, you have a situaiton that can't be good."

"The demand for real estate partnership securities today is greater than it ever has been," Chase said. However, he added, "the investor as well as the accounting and legal communities have become more knowledgeable . . . I believe the investor today is shopping around and looking at two or three [possible] transactions before he makes a choice."

There is such a thing as a bod deal, however. And SEC registration doesn't guarantee success, in public offerings.

"All the SEC really demands is full disclosure," McLennan said. "The offering can be the most wildly speculative thing in the world and all the SEC says is that in that torrent of verbiage [in the prospectus] there has to be full disclosure."

Said Balcor's Fortunato: "The single most important thing [for the investor] is the credibility of the sponsor. The general partner calls all the shots. His track record and integrity by far are the most important things."

Experts advise asking the general partner for references and checking with local bankers, lawyers and real estate brokers who don't themselvess syndicate but are familiar with the general partner's past performance.