Over the last decade the benign image of the antiquities collector has given way to a far more sinister one. Once cast as generous lenders and donors the lifeblood of American museums such collectors are now seen as central cogs in a conspiracy to move artifacts looted from foreign soil into museum display cases. "The impression is we're in a cabal with bandits," said Michael Conforti, director of the Sterling and Francine Clark Art Institute, in Williamstown, Mass., referring to damaging evidence that has emerged in an Italian crackdown on the illegal antiquities trade. Museum officials like Mr. Conforti argue that the public has forgotten why collectors are so important and, by implication, what museums are all about. To make their position clear, they have drawn up new ethical guidelines for loans of antiquities that vigorously defend the museum-collector relationship. The guidelines, to be published on Monday by the Association of Art Museum Directors, state that museums are duty-bound to examine any legal or ethical issues relating to ancient artworks that they borrow from a private collector. Even so, the new guidelines add that a hazy provenance need not stand in the way of a loan if the work has significant value. "Archaeological material and works of ancient art for which provenance information is incomplete or unobtainable may deserve to be publicly displayed, conserved, studied, and published because of their rarity, historical importance and aesthetic merit," that provision states. Such language is likely to raise the hackles of archaeologists, who argue that works lacking a well-documented history of origin and ownership often come from illicit sources. By displaying such works and giving them the imprimatur of a public institution, they say, museums are increasing the demand for and the value of looted objects. The issue is particularly delicate as foreign governments press claims on works from two of the most prominent private American collections, the Lawrence A. and Barbara Fleischman collection and the Leon Levy and Shelby White Collection. The J. Paul Getty Museum displayed the Fleischman collection before acquiring it in 1996, and some works from the Levy-White collection are on loan to the Metropolitan Museum of Art. This week, the Met said Ms. White, who is a trustee of the museum, hoped to begin talks with the Italian government about works in her collection, including one on display at the museum. The new guidelines make clear that the association is reluctant to set general standards that may affect the traditional rapport between art lenders and the museums they support. Decisions on loans ultimately rest with museum boards and directors, according to the recommendations, which are nonbinding. "These things need to be decided on a case-by-case basis," said Mary Sue Sweeney Price, the association's president. The new standards complement guidelines on acquisitions adopted in 2004, she added. The guidelines seem to reinforce a gap between the museum association's position and that of many European museums, which have taken a position against undocumented antiquities in recent years. The British Museum, for example, requires that any proposed acquisition or loan be accompanied by proof that it left its country of origin before 1970. But American museum directors involved in drafting the guidelines said they did not want important works to be excluded from the public domain simply because of scant documentation. In the antiquities trade, they noted, the origin, or find spot, of a large majority of pieces is unknown, and many works that remain in private collections do not appear in publications. Lending such a work to a museum, they say, can rescue it from obscurity and contribute to research. Some even argue that a loan can ease the way for potential claims by a foreign government. "If it goes on view with other like objects, then scholars get to see it and study it; the public gets to come; the claimant, if there is one, gets to know where it is and file a claim," said Timothy Potts, the director of the Kimbell Art Museum in Fort Worth and the chairman of the task force that drew up the guidelines. "Who has lost in this process?" Some museum directors argue that the current wave of antiquities claims against museums and collectors actually resulted from active efforts by museums to display the works and publish articles about them. "That's the ultimate irony," said Peter C. Marzio, the director of the Houston Museum of Fine Arts and another member of the task force. "Most of these claims would never have been made if the institutions and the collectors have not been this open and transparent." Italian investigators have acknowledged in recent months that museum publications, auction catalogs and even visits to museum galleries have been useful in tracing illicit works. But they also noted that their success also hinged on a rare cache of evidence discovered in a 1995 raid on Swiss warehouses belonging to an Italian art dealer. Since the new guidelines leave it up to foreign governments to track down such evidence, "the burden of proof is in the wrong place," said Richard M. Leventhal, the director of the University of Pennsylvania Museum of Archaeology and Anthropology, which does not acquire or display works that have surfaced since 1970. Similarly, the American Journal of Archaeology says it will not serve as the primary publication for newly surfaced archaeological material unless the objects in question are documented as having emerged before 1973, the year the Archaeological Institute of America endorsed a Unesco agreement regulating the circulation of cultural goods. Museum leaders, meanwhile, suggest that legal claims may be a natural hazard of collection building. Peter Morrin, the director of the Speed Art Museum in Louisville, Ky., and former president of the Association of Art Museum Directors, said the provision was a nod to the dangers that museums face in accepting antiquities loans. "It's not a loophole," he said. "It's a recognition that we are taking great risks." Indeed, many museum officials said that a changing American legal climate for patrimony claims by foreign governments had left museums much more exposed than before. These days, they added, few institutions are likely to borrow or acquire works that have any serious questions about them. This week, the Met signed an agreement for the return of 21 objects that Italy asserted had been looted from its soil. Italy has also made major claims against the Getty, whose former antiquities curator, Marion True, is on trial in Rome for illicit trafficking in antiquities. Museums are struggling to figure out just what is still all right to show in their galleries. "You are going to see museums growing legal staff and getting more bureaucratic," Mr. Marzio said. For Mr. Potts, an archaeologist by training, the recent attention to the role of collectors and museums in fostering the destruction of archaeological sites is misplaced. The real issue, he argued, is insufficient incentives in countries like Italy and Greece for discoverers of objects to report their finds. "So much of the pressure is focusing on the wrong end of the chain," he said. "I think there should be much more done to stop the looting at its source."