Paolo Ferri, an Italian prosecutor who is investigating the purchases of antiquities by major American museums, has hit hardest at the J. Paul Getty Museum in Malibu, Calif., which in recent decades rapidly built up an impressive collection of Greek and Roman art.

The Getty's troubles — compounded last month by legal action from Greece for the recovery of four works — offer a useful lesson for museums and collectors. But it is unfortunate that a chief target of the prosecution is the Getty curator who has done the most to clean up the practices of her institution in this murky trade, Marion True.

According to the Italian court, for two decades before 1995 the Getty bought many previously unknown antiquities that had been looted from archaeological sites. Such works were laundered by the antiquities market, and consequently almost nothing is known (at least by the public) about where they came from or what purposes they served.

Some of these are among the most important discoveries of the period, and the loss of information about their origins is painful. The Getty's controversial "Morgantina Aphrodite" is an extremely rare example of the sort of cult statue that once stood within a Greek temple. While, as some have asserted, this remarkable work may come from Morgantina (a site in Sicily where I serve as co-director of excavations), no proof of its origin is known, and its subject is just as uncertain. The market destroyed the evidence.

For this and countless works, many questions remain unanswered: Where were they found? What artists and patrons conceived them? When were they used? Most such works of art were clumsily excavated at night (the Aphrodite was badly damaged in the process, as must have been the spot from which it was taken), then absorbed into the art market stripped of their earlier history, including any record of ownership in antiquity — just the sort of information about provenance that museums are expected to go to lengths to preserve.

The recent revelations about the Getty's dubious purchases are old news to archaeologists who worked at classical sites in the Mediterranean in the 1980s and early 1990s; we regarded the museum as a powerful stimulus to the illegal market. For the past decade, however, the Getty has prohibited the purchase or acceptance as a gift of any work whose existence is not documented before 1995.

Undocumented antiquities are very likely to have been pillaged. By adopting a concrete date before which the object had to be known, the Getty has distanced itself from the illicit market, and the distance will increase with time.

The pre-1995 publication rule is vital because dealers often have invented fake pedigrees for the works they sell. The Getty's present acquisitions policy is owed to True, its former curator of Greek and Roman antiquities. The Getty policy is arguably the strongest of any major American museum, and as far as we know, it has not been violated.

Other museums, including the Metropolitan Museum of Art, the Museum of Fine Arts in Boston and several major university collections (Princeton's and Harvard's among them), instead follow the policy adopted by the Association of Art Museum Directors, which allows the purchase of undocumented antiquities if the museum believes acquisition is justified. The problem here is that objects newly on the market with no known history are almost certain to have been recently pillaged. If dealers revealed the origins of such works they could not possibly be sold. Photographs seized in a 1995 police raid on the warehouse of one dealer, Giacomo Medici (who already has been convicted through the efforts of Ferri) show Italian soil still clinging to vases now in American collections, including the Metropolitan. Most archaeologists, of course, would prefer an acquisitions policy that is even stronger than the Getty's — one that would require proof that the object was documented much earlier than 1995. Some advocate the symbolic date of 1970, when the UNESCO convention on illicit trade in cultural property was approved. A more rigorous choice would be the date of the relevant legislation protecting antiquities in the country of origin (in the case of Italy, June 1, 1939). Either way, choosing a date is essential.

The pillaging of the human past is a problem the world over, hardly limited to the Mediterranean. To reduce it, all countries that have antiquities at risk should police their historical sites effectively and create programs that teach citizens the value and community importance of local remains. The international trade can also be discouraged by import bans. The UNESCO convention allows the United States to sign bilateral agreements with countries where pillaging is rampant, banning entire categories of objects at risk. Nine such agreements are now in force with countries in Central and South America, Africa, the Mediterranean and Asia. (The agreement with Italy is up for renewal.)

In the end, however, the law can do only so much, and as legitimate custodians of human achievement, the museums should adopt higher standards in building collections, cutting their ties with the illegal trade.

Ferri's outrage at the looting of Italy's heritage is justified. By laying bare the archives and warehouses of major dealers, he has revealed corruption at the core of the market.

But in prosecuting Marion True, he has used decades-old evidence against a curator who brought needed reform to the Getty Museum, and I can only hope the Italian courts recognize the good she has done.

If there is one major lesson to be learned from Ferri's investigations, it is that collectors and museums, in America and around the world, must take into account not just the aesthetic value of the objects they acquire but also the ethical and legal consequences of their acquisition policies.