The sale followed news earlier this week that Grey doubled its profits over the past quarter to $10.8m, on the back of strong performance in Europe, as revenues rose 13.8% to $364m.
According to a filing with the US Securities and Exchange Commission, Brookside has reduced its stake in Grey from 7.2% to 3.7%.
Brookside, which is linked to private investment firm Bain, was Grey's third largest shareholder after Ariel Capital Management and Ed Meyer, the chairman of Grey.
The company offloaded its shares at a price of $874.50 per share after they had dropped back from a peak of $999, to which they had surged after initial reports that the US's third-biggest advertising firm was putting itself up for sale.
Grey is expected to have a price tag in excess of £700m. Two rival advertising holding companies, WPP Group and Havas of France, and two private equity firms, Hellman & Friedman and Kohlberg Kravis, Roberts have expressed an interest in buying the group.
Hellman & Friedman is reported to be working with former Young & Rubicam chief Mike Dolan on its bid to buy Grey, which owns the advertising agency Grey Worldwide, media agency MediaCom and the PR networks APCO and GCI Group.
Sir Martin Sorrell, the WPP chief executive, has already told reporters that the company's major clients have given the OK for the British group to make a bid. It is currently carrying out due diligence.
Grey's share price closed yesterday at $880.
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