The Italian government has successfully brokered deals with American museums and private collectors for the return of what it says are looted antiquities. But it is finding the Ny Carlsberg Glyptotek, an art museum in Copenhagen, harder to crack.

Talks with the Glyptotek have dragged on for months, even though "the presuppositions for the negotiations are identical to those that were carried out with the Americans," said Maurizio Fiorilli, a lawyer for the Italian state involved in the negotiations. The Glyptotek, however, has "adopted a very different attitude," he said.

At the core of the dispute are Etruscan and Greco-Roman objects that the Glyptotek bought from Robert Hecht, an American antiquities dealer now on trial in Rome, where he is accused of receiving and selling stolen artifacts and conspiracy in the antiquities trade. He denies any wrongdoing.

The Italians have used evidence from Mr. Hecht's trial, and from the trial of the antiquities dealer Giacomo Medici, who was convicted of receiving and smuggling archeological artifacts, to persuade several American institutions — including the Metropolitan Museum of Art in New York and the J. Paul Getty Museum in Los Angeles — to return objects to Italy on the suspicion that they were illicitly excavated. (Mr. Medici is appealing his conviction.)