By Robin Sidel And Liz Hoffman 

Investment firm ValueAct Capital Management LP has taken a roughly $1 billion stake in American Express Co., according to a person familiar with the matter, the latest example of an activist investor targeting a blue-chip firm.

It isn't clear at this point if San Francisco-based ValueAct plans to agitate for changes at the big New York-based card company, which has been led by Chief Executive Kenneth Chenault since 2001.

"ValueAct is a well-respected firm," AmEx said in a statement. "We have been speaking with them, as we do with other investors, and look forward to continuing a constructive dialogue. At American Express, we are focused on building long-term value for shareholders, and are always open to the views and perspectives of our investors."

AmEx has seen its stock price fall 13% this year due to two high-profile losses that are expected to take a big bite out of earnings for at least the next year.

The move by ValueAct, first reported by Bloomberg, sent shares of AmEx up more than 6%, to $80.05, on the New York Stock Exchange.

Write to Robin Sidel at robin.sidel@wsj.com and Liz Hoffman at liz.hoffman@wsj.com

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