The writ, issued by White & Case, a New York law firm, alleges he breached his contract and fiduciary duties, and misappropriated confidential and proprietary information on group companies.
A spokesman for Saatchi & Saatchi said the reference to confidential information related to files from Mr Muirhead's New York office.
Earlier this month the company used a threat of legal action to force Maurice Saatchi - deposed as company chairman in December - to return 15 filing cabinets removed from his office and housed briefly at his brother's art gallery in St John's Wood, north London.
The company is keen to discuss the episode with his elder brother, Charles, who is honorary president of Saatchi & Saatchi. "We are continuing to review the situation," a company spokesman said yesterday.
Mr Muirhead yesterday claimed he had only personal files containing items such as bank statements.
He was bitterly critical of the second writ to be issued against him by the company in less than a week. "I am advised by my lawyers that this is a totally fanciful claim. It contains a number of unsubstantiated and untrue allegations without any evidence supporting them.
"It will be vigorously defended. The figure of $50m has no basis and seems designed only to attract newspaper headlines and to intimidate me and my family," he said.Reuse content