Napster got a new enemy Tuesday, and this time it's one of its own. Music subscription site EMusic.com has begun using software it claims can spot any of its 140,000 songs that are being traded illegally by Napster users. Warnings will be first sent through instant messages, giving the person 24 hours to stop swapping the song. Napster has apparently agreed to block the person's account if illegal trading persists. If users find a way back into Napster through a different Internet Protocol address, EMusic will then try to block the person's Internet access. It is the same type of arrangement Napster has reached with musician Dr Dre and the rock group Metallica. It is the first, however, to be reached between Napster and another Web site, according to EMusic chief executive Gene Hoffman. "The reality is, we're drawing a line in the sand right now," Hoffman said. Ric Dube, an analyst with Webnoize, called the move a precursor to a possible lawsuit that would pit two pioneers, Napster and EMusic -- the first and so far only large-scale, subscription-based MP3 download sites -- against each other. "This is a warning shot," Dube said. "They are saying they have the technical ability to track their music. " Hoffman estimates that between one-half to three-quarters of the EMusic catalog is traded on Napster. A Napster representative did not return phone calls for comment Tuesday morning. A statement was expected from the company this afternoon. EMusic's "acoustic fingerprint" technology is capable of tracking songs traded on other file-swapping applications, such as Gnutella and Freenet. However, Hoffman said that EMusic is only targeting Napster -- for now. The situation puts Hoffman in the unusual position of siding with the Recording Industry Association of America (RIAA), which is suing Napster for copyright infringements. The RIAA recently settled a similar suit against MP3.com. "It's always weird being in the same camp as the RIAA," Hoffman said. "We don't fully agree with them -- some of their stances are off-base." Napster recently reached an agreement with Bertelsmann A.G. , which owns BMG, one of the five record companies involved in the lawsuit. According to the agreement, BMG will dismiss its part of the lawsuit if Napster develops a subscription model, with a possible fee of less than $5 a month. Napster has claimed fingerprinting software isn't viable, a major key to the company's defense against piracy allegations from all five major record labels accusing it of piracy and copyright infringements. But Hoffman said that claim is "disingenuous". "It took us about three weeks and two engineers," he said. "This is not rocket science, unfortunately." EMusic officials said the company made its decision after a few months of fruitless talks with Napster executives, imploring them to use the "acoustic fingerprint" technology. EMusic originally gave Napster until Friday to voluntarily use the fingerprinting software. However, that deadline stretched until Tuesday morning, when Napster officials said "they were still looking at it", according to Hoffman. Take me to the MP3 Special Have your say instantly, and see what others have said. Click on the TalkBack button and go to the ZDNet News forum. Let the editors know what you think in the Mailroom. And read what others have said.