An enigmatic international businessman has acquired a major interest in the proposed Tregaron development, fueling fears among surrounding residents that the last major undeveloped tract in Northwest Washington may eventually be sold for high-density housing instead of the restricted town house construction now being discussed.

Shaul N. Eisenberg, whose multibillion-dollar business empire is based in Israel and extends to nearly 40 countries, invested in the Tregaron project last year through a corporation set up in the Netherlands Antilles called Catopale Investments, N.V., according to Paul Berger, a Washington lawyer who represents Eisenberg in this matter.

Catopale holds a 37.5 percent interest in the Tregaron Limited Partnership, the local developer, with an option to acquire up to a 62.5 percent interest, according to partnership documents filed with the D.C. Recorder of Deeds in January 1981.

The Tregaron development company said it intends to go forward with plans to build 120 luxury town houses -- already twice as many as opponents of major development want -- if the D.C. Zoning Commission approves the project. But word that a foreign investor has entered the picture has raised new concerns among area residents that the developer may want a zoning change to allow 120 town houses in order to enhance the value of the land, and not to build town houses.

"If Eisenberg does have control, it adds fuel to the assumption that the property will be resold, because he's an entrepreneur, not a developer," said Sheldon Holen, a dentist and president of the 300-member Friends of Tregaron, which opposes major development of the site.

E. David Harrison, general counsel to Tregaron Limited Partnership, last week denied any knowledge of Eisenberg's involvement. He said, however, that the developer was serious about going ahead with building the $330,000-apiece town houses if the necessary zoning change is approved.

"This is not just an exercise -- we're not playing games here," Harrison said. "No one would have spent three years and hired the best architects and consultants . . . unless they were planning to go ahead."

"I can't guarantee anything, but this group is serious about going ahead," he added. "If we went into a depression and the whole economy went to hell and you couldn't build a house -- that's possible but highly improbable. But I know of no reason they wouldn't go ahead with this."

Tregaron, a heavily wooded tract and a designated historical landmark, was sold in November 1980 for $4.6 million by the heirs of Joseph E. Davies, a wealthy Washington lawyer and ambassador to Moscow before World War II, under a court-ordered settlement.

The property is located southwest of the intersection of Connecticut Avenue and Macomb Street NW. The Washington International School acquired the estate's Georgian-style mansion and about six acres of land.

The remaining 14.6 acres was purchased by the Tregaron Limited Partnership, formed by two lawyers and longtime business associates, Alan R. Novak of Washington and Raymond A. Lamontagne of Wilton, Conn. The two men also are involved in an 800-are town house development called Valeria, located outside New York City.

In a recent telephone interview, Lamontagne said that he and Novak were forced to seek out additional investors when their partnership ran short of capital. Lamontagne said that he persuaded a New York-based company called Wood River Capital Corp. to purchase a 13 percent interest in the partnership, while Novak lined up Catopale Investment. He said he was not sure who was actually behind Catopale.

"We needed money," he said. "I got Wood River. Al got Catopale."

Novak was out of town over the weekend and could not be reached for comment. Last week, he referred all questions about the development to Harrison.

Eisenberg was profiled in a 1981 Business Week article as a businessman of enormous wealth, diversity of business interests, and a proclivity for secretiveness in his dealings. He owns a worldwide network of corporations. His largest company, United Development Inc., was set up in Panama in 1960.

Eisenberg essentially acts as a middleman, according to reports on his dealings, assembling consortiums of manufacturers, builders and banks to construct industrial facilities of all types.

During the last two or three years, Eisenberg has begun to expand his activities in the United States, including real estate investments in Denver and Washington, according to Berger, his lawyer.

Last year, Eisenberg sold his interest in a block-square property on Pennsylvania Avenue, between 10th and 11th streets, that is part of the proposed Lincoln Square office building complex. The same year he bought into the Tregaron development project.

Cleveland Park residents who for years used the sloping estate as a park have vigorously opposed the effort by the Tregaron partnership to build a development of town houses.

Under the current R-1-A zoning, the developer may build about 80 detached single-family homes on lots of at least 7,500 square feet, but that would necessitate cutting down many of the trees and spoiling the topography.

The developer has asked the zoning commission to upgrade the zoning to R-1-B, to permit construction of 120 town houses -- twice the number acceptable to Friends of Tregaron. In return for the zoning change, the developer has promised to cluster the town houses on a fraction of the property, to preserve most of the trees and create a public scenic easement.

An earlier proposal to build 189 town houses ran into opposition from the District of Columbia's historic landmarks commission.

The D.C. Office of Planning and Development gave qualified support of the latest proposal at a zoning commission hearing last week. James O. Gibson, director of the office, noted in a report that the proposal would leave 70 percent of the site as green space, take care of storm water runoff, meet environmental concerns and assure good visibility on the soutern side of the mansion.

But the project has been opposed by the Joint Committee on Landmarks, the local Advisory Neighborhood Commission, the Friends of Tregaron and City Council member Polly Shackleton (D-Ward 3).

Shackleton and local residents contend that the proposal would create serious traffic and parking problems along bordering Macomb and Klingle streets NW and adversely affect the neighborhood ambiance.

"The high density of this particular proposal . . . would adversely and irreversibly affect the historic qualities of the Tregaron Estate and would be destructive of the character of widely separated single-family homes in the surrounding neighborhood and quiet residential streets," Shackleton told the zoning commission.

The plan also was criticized by the National Capital Planning Commission, which has urged that the project be scaled down further to preserve the landmark qualities of the site.

The commission will hold another hearing on the proposal at 7 tonight at the Capital Memorial Seventh-day Adventist Church, 3150 Chesapeake St. NW. Several of the opponents, including Friends of Tregaron, will make presentations.

Said Holen: "The neighborhood has been fighting it not because we want this private property as our public park, but because we want the development a lot less dense."