America's music and movie industries are appealing against a court ruling stopping them from suing companies which offer file-sharing software.
Music and media firms say file-swapping threatens their industries
In April, a federal judge ruled the companies behind Grokster and Morpheus could not be held responsible for their users' copyright violations.
Now trade groups including the Recording Industry Association of America (RIAA) and the Motion Picture Association of America (MPAA) have filed an appeal at a San Francisco court.
They accuse the companies of trying to make money out of copyrights they do not own.
In a submission filed to the 9th US Circuit Court of Appeals, they said Judge Stephen Wilson departed from well-established copyright law when he ruled Grokster Ltd and StreamCast Networks - which distributes Morpheus - were not responsible for their users' actions.
"The decision dramatically redrew the law... to set near-impossible standards for liability in an online environment," they said.
"This case... is about the conduct of business that intentionally misuse commonly available internet technology to profit from copyrights they do not own."
The judge acknowledged that while the companies may have deliberately structured their businesses to avoid liability while profiting from the illicit use of their software, he ruled there was no evidence they could supervise and control what their users did.
The trade groups say the decision "makes a mockery of copyright law", allowing people to form companies which profit from others' work, but without any consequences.
RIAA president Cary Sherman said: "These are businesses that were built for the exclusive reason of illegally exchanging copyrighted works and they make money hand over fist from it. The Court of Appeals should hold them accountable."
StreamCast chief executive Michael Weiss said the main issue was that whether file-sharing developers should be held liable for developing a product with many legitimate uses, just because some people misuse it.
"We expect to prevail, and if we do not, we will take this to the Supreme Court if we must," he said.
Grokster president Wayne Rosso said: "We clearly have the law on our side, something the plaintiffs obviously have a difficult time accepting."
The RIAA has threatened legal action against individuals who swap music on the internet, but recently said it would only target those who downloaded "a substantial amount" of music.