Facebook founder Mark Zuckerberg says emails and a signed document that form the basis of a New York man's lawsuit claiming part ownership of the social networking phenomenon are fakes.
In papers filed in federal court in Buffalo, Zuckerberg's lawyers ask for expedited access to the materials and to examine plaintiff Paul Ceglia's computers and electronic media.
Mr Ceglia says he has a 2003 business contract and emails that prove he bought into Zuckerberg's idea for Facebook when Zuckerberg was a Harvard student.
Facebook lawyers call the contract "a cut-and-paste job" and the emails fakes.
Ceglia's lawyer Dennis C Vacco says he welcomes expedited discovery but disagrees with Facebook's characterisation of the documents.
Facebook has more than 500 million users worldwide. Zuckerberg's net worth is estimated at 13.5 billion dollars (£8.25 billion).