On an easel near a leaded window overlooking the Swiss lake of Lugano, "The Madonna with the Bunch of Grapes," a 16th-century painting by Lucas Cranach the Elder, was getting a last-minute cleaning in the conservator's studio.

Soon, packed and looking her best, "The Madonna" would be heading for America. Traveling with her, on six different planes, would be 56 co-habitants of the Villa Favorita, all part of the Thyssen-Bornemisza Collection of old master paintings. Destination: the National Gallery of Art, where, on November 18, they would begin a two-year, nine-museum American tour.

Outside the villa, visitors strolled from the entrance gate down a long path shaded by palms, magnolias and giant cypresses planted by a former owner -- and amateur painter --the Prussian Prince Leopold. In this idyllic Alpine setting, where the only sounds are water lapping against vine-covered embankments, ferries ply the lake between its Swiss and Italian shores. A discrete blue and red chain separates the gallery wing from the now-shuttered residence, a polite reminder that this is a private place.

It is, in fact, the home of the second Baron Thyssen-Bornemisza, his fourth wife and their 6-year-old son when they are not at their principal residence, an English country house near Oxford, or at the beach house in Sardinia, or at various other pieds-a-terre in London and New York, or in their pied-au-ciel, a Falcon 20 jet.

But this villa on a lush hillside just outside Lugano in the southernmost Swiss-Italian canton of Ticino has been a mecca for art lovers ever since it opened to the public after World War II. Today it houses some 500 paintings and sculptures dating from the 14th through the 19th centuries -- considered the finest collection of old master paintings still in private hands, unless you call the Queen of England a private collector.

The only other private old master collection in the same league was inherited by the Prince of Liechtenstein, just over the Alps. But he has been selling, not buying. "His collection may be larger, but mine is better rounded," says the baron, who has added some 200 paintings, including examples from the Italian, Spanish and late French schools which did not interest his father.

Two-thirds of the collection at Villa Favorita was assembled by the present baron's father, the first Baron Hans Heinrich Thyssen-Bornemisza (who got his title when he married an Hungarian baroness). In 1932 he purchased this Romantic-Romanesque 17th-century villa and six years later added a new gallery wing to house the old master collection he had so swiftly assembled in the previous dozen years. Bernard Berenson, a smattering of Rothschilds and J. Paul Getty turned up for the opening, according to a guest book still kept in the gallery. Visitors as varied as Marshall Mannerheim, Stavros Niarchos and collector Norton Simon and his actress wife Jennifer Jones have followed, along with an army of scholars.

The first baron dedicated the gallery to his father -- steel, iron and banking magnate August Thyssen, founder of the family fortune and sometimes called the Andrew Carnegie of Europe. Nothing was spared, and though the galleries are of comfortable human scale, they are resplendent with colored marble floors and doorways in the neo-classical style, rich red damask and velvet-covered walls, huge pots of flowers inside and sweeping garden vistas outside -- galleries created in the days before the big, empty white room became derigueur for showing art.

From the first glimpse of the early Italian Renaissance paintings, Gothic ivories and stone and wood carvings from Rheims and Ile de France, it is clear that this is a collection of spellbinding scope. Throughout, sculpture and decorative arts contemporary with the paintings -- chests, clocks, chairs, tables, tapestries and rugs -- have been thoughtfully joined in harmonious, reinforcing installations.

In the 20 galleries that follow, there are some paintings chiefly of interest to scholars, but many more that stop the visitor in his tracks. One such is the small portrait of Henry VIII by Holbein, possibly the only extant portrait of the king by the artist's own hand, painted just before Henry's marriage to Jane Seymour in 1536. It will not be making the trip.

In the third gallery, past several paintings by Cranach Pere and Jeune, a great Hans Memling portrait of a young man is installed with a mirror to show a still-life painted on the back. This is the signature painting of the traveling show and is emblazoned on the poster and the catalogue cover.

In that same small room -- a veritable wunderkammer of Flemish art -- are several other jewel-like early 15th-century paintings by Rogier van der Weyden, Petrus Christus and Jan van Eyck, whose two trompe-l'oeil panels in grisaille would put today's photo-realists to shame.

Moving on past two Rembrandt portraits and several Dutch genre and landscape paintings, a "Portrait of a Family" by Frans Hals looms large. One of this artist's finest group portraits, it once graced the New York mansion of financier Otto Kahn.

It was also the Kahn collection from which the first baron purchased, during the hard times of the 1930s, another of his greatest acquisitions -- and a highlight of the traveling show - "Young Knight in a Landscape" by the early 16th-century Venetian painter Vittore Carpaccio, a tour de force of flower and animal painting.

There is much more, including a marble "Pieta" by Bernini (the only sculpture by this artist still in private hands), and yet another masterpiece which unfortunately left America in the '30s -- this one from the Pierpont Morgan Library in New York. It is a profile portrait of Giovanna Tornabuoni by the 15th-century Florentine painter Domenico Ghirlandaio and is the favorite painting of the present Baron Thyssen-Bornemisza. It will not travel. Overall, however, Thyssen has not held back, and what is coming represents much of the best he owns.

The paintings in the traveling exhibition were selected at least in part, to show how remarkably the collection has grown since the death of the present baron's father in 1947. This growth has been especially impressive in a period when such masterpieces were extremely hard to come by, even if one could afford the six- and seven-figure price tags. Money has never been a problem for Thyssen, considered among the wealthiest men in the world.

'Within the context of that market, Thyssen has done brilliantly," says Princeton professor Allen Rosenbaum, who wrote the catalogue entries. "Who would think that as late as 1971 one could add a painting like a Duccio from the maesta altarpiece in Siena?" This painting was acquired privately in 1972 from the John D. Rockefeller II collection. The National Gallery owns a panel from the same altarpiece, and considers it one of its great treasures.

"Also, the brilliant little Guardi Turkish scenes may not be in the same league with Duccio, but they are the finest of their kind, dazzling little paintings," says Rosenbaum.

Thyssen has also added one of the most powerful portraits in the collection -- the Antonello da Messina "Portrait of a Man in Black," which stares straight at the viewer, past four centuries. It is Carter Brown's favorite. There are other important additions, including two "Annunciation" paintings, early and late, by El Greco, two Goyas and two Zurbarans.

But it is testimony to the baron's eye" that he has also purchased several stunning works by lesser-known artists, such as "Still Life with Nautilus Cup" by 17th-century Dutch painter Willem Kalf, which features a sparkling, partially peeled lemon that actually makes the viewer's tongue pucker.

Scholars have been less enthusiastic about a handful of other recent acquistions, including a Rembrandt self-portrait, which appears to have fared rather badly during its lifetime. "There has been some hair-splitting," admits Rosenbaum, "but the fact is that any museum in the world would be happy to have any of the paintings in this collection."

If Thyssen has his wish, the collection will remain private and will not be institutionalized, as have most of the other old master collections in the world, usually either for reasons of ego or taxes.

The collection, in fact, only exists intact today because of the current baron's tenacity. After his father's death, he was forced to divide the paintings, along with the business holdings, between four heirs. He bought back what he could. Other paintings, however, were inevitably lost to a voracious market, including a Durer "Madonna & Child" purchased by American collector Samuel Kres and now hanging in the National Gallery.

One important El Greco, it is said, came back to the villa as part of a seven-page divorce settlement with Thyssen's second wife a former Dior model. He let her keep the Ferrari with the gold ignition key, four race horses, an airplane, an island in the Caribbean and 70 acres near Versailles. All he asked in return was his El Greco. He got it.

A small jewel in the collection, the rare Petrus Christus "Out Lady of the Barren Tree," came back by a more circuitous route. It had been promised as a legacy by a German aunt, but she and the baron quarreled, and in any case, the West German government had declared the painting a national treasure, which meant that it could no longer leave the country.

One day the aunt impulsively gave the painting to a high West German official, who, some years later, in need of cash, sent it to Switzerland and sold it. No questions were asked, no doubt because of his high rank. The baron heard that the painting was on the market, bought it and returned it to La Favorita.

Five years ago, Thyssen also added an American conglomerate to his collection -- Indian Head, which produces glass containers, car parts, data systems and more little yellow school buses than anyone else in the country. According to his associates, Indian Head is worth more than $500 million. According to the Baron, it represents roughly 50 percent of the family's current business interests, which also embrace shipbuilding, shipping, gas distribution and "other port activities" in Europe.

So why is the tenacious and acquisitive baron now allowing his fragile treasures to go touring the United States?

"It seemed only fitting," he said, "given my new involvement with America." So far as art is concerned, it is a rather profound involvement already. He has a Zarburan on loan at the Metopolitan Museum, six paintings at the Federal Reserve Board building, and jointly owns a pair of the most expensive silver tureens ever sold at auction with the Cleveland Museum of Art. Other museums here have asked to borrow works from his little sidline collection of 500 modern paintings now touring Australia.

The baron has obviously already discovered America. Now the time has come for America to discover him.

The exhibition was organized by the International Exhibitions Foundation of Washington, and the selection made by former National Gallery director John Walker, who wrote the introduction to the catalogue. The Washington showing will continue through February 17, after which it will travel to Detroit, Minneapolis, Cleveland, Los Angeles, Denver, Fort Worth, Kansas City (Mo.) and New York City.