Facebook sued four law firms on Monday over their involvement in a case challenging Mark Zuckerberg's ownership of the social media company.
The suit was filed in New York against firms—DLA Piper, Millberg LLP, Paul Argentieri & Associates and Lippes Mathias Wexler Friedman LLP—which were involved with Paul Ceglia's suit against the company.
Facebook shares were about 1 percent higher in mid-day trading.
Ceglia sued Facebook and Zuckerberg in 2010, claiming he owned a stake in the company. Federal agents arrested him in late 2012, alleging he manipulated a contract between himself and Zuckerberg. He is reportedly due to go on trial soon on those charges.
Ceglia's suit against Facebook was dismissed earlier this year.
Lippes Mathias Wexler Friedman said the allegations were meritless. "Any claim of malicious prosecution on the part of the plaintiff is not only false, but is, in and of itself, a malicious prosecution...," the firm told CNBC in a statement.
Representatives for the other three firms were not immediately available for comment.