Facebook founder Mark Zuckerberg has said that an alleged contract and e-mails that a New York man claims entitle him to a 50% stake in the social networking site are "forgeries".
Paul Ceglia says that Mr Zuckerberg signed a contract in 2003 which gave Mr Ceglia half-ownership of Facebook.
In a court filing, Facebook and Mr Zuckerberg demand that Mr Ceglia turn over the alleged contract and e-mails.
Lawyers representing Mr Ceglia disputed Facebook's claims.
"Those so-called expert opinions have been provided without examining the actual contract which is at issue in the case," said Dennis C Vacco, who is one of Mr Ceglia's attorneys.
In a filing made at the US District Court in Buffalo, Mr Zuckerberg said he provided web development services in 2003 for StreetFax, a business Mr Ceglia was trying to start at the time.
He said he signed a contract drafted by Mr Ceglia, a wood pellet salesman, which referred only to the work he did for StreetFax.
"Zuckerberg and Ceglia never discussed Facebook and they never signed a contract concerning Facebook," the filing said.
"The contract is a cut-and-paste job, the e-mails are complete fabrications, and this entire lawsuit is a fraud."
Facebook and Mr Zuckerberg hope to use forensic testing to show that the documents are fakes.
Facebook is privately owned but estimates of its worth range between $50bn (£30bn) and $76.4bn (£46.7bn).
Twins Cameron and Tyler Winklevoss also claimed that Mr Zuckerberg stole their website idea while they were all students at Harvard.
In 2008 they reached a settlement which gave them $20m in cash and $45m of stock valued at $36 a share.
They have since unsuccessfully tried to reopen their case against Facebook, claiming that the company concealed information and they should have received more shares.