VINCI, Italy A 19th-century German School portrait that sold for 21,850 at a Christie's auction in 1998 has now been attributed to Leonardo da Vinci by some art and scientific experts. Da Vinci's "Marriage Portrait of a Young Woman." A spokesman for the mixed-media portrait's Swiss owner, who wishes to remain anonymous, said offers to buy it have already started pouring in. He said the top bid so far was more than 50 million, by an intermediary acting on behalf of a Russian. But the attribution has not gone unchallenged. The 13-by-9.4-inch work which might be a betrothal portrait did not cause a furor when it went on sale at Christie's in New York. At the auction, it was bought by a dealer based in the United States, who sold it last year. If it is in fact a Leonardo, skeptics say, it went unrecognized by experts at the auction house, as well as the specialized dealers who attended the sale, including the one who bought it. "The market is a fairly efficient place," said Hugh Chapman, assistant keeper at the department of prints and drawings at the British Museum in London. "This would be an amazing miss." Still, there are those who believe the work is a genuine Leonardo. "This profile is almost too beautiful to be true," said Alessandro Vezzosi, director of the Museo Ideale Leonardo da Vinci, pointing to a photograph of the work that now hangs here. Scientific studies aside, he said, "the iconography and the aesthetic speak clearly" that it is a Leonardo. Mr. Vezzosi included the portrait in a Leonardo monograph he published in July. Christie's said in a statement that it "cannot comment on this particular work until it has been the subject of comprehensive and conclusive academic and scientific analysis." Other experts point out that modern connoisseurship a convergence of wide-ranging technical examinations and the expert's eye remains an imperfect science. And building consensus around an attribution can be a long and challenging process. The story of how a Swiss collector bought a pretty portrait in January 2007 and ended up with a work that might be by a Renaissance master is a "rags-to-riches story, except that the owner is not exactly in rags," said Peter Silverman, a Canadian collector who is a friend of the owner. Eighteen months ago, Mr. Silverman said, the Swiss collector showed the portrait to him, and he was the first to suspect that his friend might have made an amazing investment. "I saw it, but I didn't dare speak the L-word," Mr. Silverman said in a telephone interview. The two collectors took the portrait to Lumiere Technology, a Paris-based company specializing in multispectral digital technology that had already digitized two works by Leonardo: the Mona Lisa at the Louvre and "Lady With an Ermine" at the Czartoryski Museum in Krakow, Poland. "The first time that the owner gave me this drawing he didn't say a thing; the author was secret," said Pascal Cotte, Lumiere Technology's chief technical officer. Though Mr. Cotte carried out a series of tests on the work for nearly four weeks, he said, it did not take him long to come up with a name. "I went to the owner and said, 'I have a feeling it's a drawing by Leonardo,' and he said, 'We're here for just that.' " In June, Lumiere announced that its examination had led to the authentication of the work as a Leonardo. Carbon 14-dating tests carried out by the Swiss Federal Institute of Technology Zurich and released this month place the work's date between 1440 and 1650. But art dealers and art historians interviewed recently had mixed opinions about the portrait. Scientific tests "can be very useful, but they can't guarantee an attribution because the first criterion is quality and that can't be discerned through mechanical means," said Jean-Luc Baroni, a London-based art dealer. Nicholas Turner, a former curator of drawings at the British Museum and the J. Paul Getty Museum in Los Angeles, saw the work last December and was struck by the left-handed shading Leonardo was left-handed as well as the physiognomy and the details. They all point in the direction of Leonardo, he said, adding, "I recommended that that avenue of inquiry be pursued among Leonardo specialists." After viewing the digitized images produced by Lumiere, Mr. Turner discounted the possibility that the work could be a fake. "Fakes fall apart, especially when you magnify them," he said. "It's difficult to produce anything that can convince people that they're genuine." Carlo Pedretti, the Armand Hammer Chair in Leonardo Studies at the University of California, Los Angeles, described the portrait as "a magnificent thing, worthy of Leonardo, even if strangely cool and lifeless." If it really is by Leonardo, he said, it would be a discovery comparable to the "early 19th-century re- establishment" of "The Lady With the Ermine" as an autograph work. That is reason enough to encourage further tests and consultations with art historians at scholarly institutions, Professor Pedretti said. "I am prepared to be fully convinced that it's a Leonardo, but let's say I'm still on the cautious side." Martin Kemp, a professor of art history at Oxford University, said, "Throw everything at it." He said that he had not seen the original, which is kept in a Swiss vault, but that based on the digital images he was "pretty convinced that it's the real thing." Other experts are more skeptical. Carmen C. Bambach, curator of drawings and prints at the Metropolitan Museum of Art in New York and one of the world's leading Leonardo specialists, said in an e-mail exchange that based on the photograph of the portrait the "work does not seem to resemble the drawings and paintings by the great master." Others have noted that it would be the first work by Leonardo on vellum. "It makes the portrait harder to compare to a more validated Leonardo," said Claire Farago, an expert in the intellectual tradition of Leonardo who teaches at the University of Colorado at Boulder. She pointed out that there were many painters working in Leonardo's circle emulating his style. In the end, Mr. Chapman of the British Museum said, "there will be a scholarly opinion, but it takes time to make its way through the system. The scholarly world needs time to digest this thing; it can't make a snap decision." Still a lot is at stake, in terms of both prestige and money. Leonardo is a star at auctions in part because little of his work goes up for sale. A silverpoint study of a horse and rider by him sold at Christie's in London in 2001 for 11.5 million. "I get on a weekly basis things that are more or less close to Leonardo that the owners are convinced are some great lost original," Professor Kemp said. Noting that a Leonardo would most likely sell in the double-digit million-dollar range, he said, "You could see why they indulge in wishful thinking." Mr. Silverman, the collector, said the owner had no immediate plans to sell the work. But until a definite attribution is made, its market value cannot be pinned down. "If one expert says yes and the other says no, it makes it unsellable, " Mr. Baroni, the art dealer, said. "No one will buy until you have certainty. If you are buying a Leonardo you want to be convinced it's a Leonardo."