A regulatory filing Thursday shows that Icahn holds a 12.98 percent stake in nutritional supplement distributor Herbalife Inc. and intends to have discussions with its management on business and strategic alternatives to enhance shareholder value, including the possibility of going private.
Herbalife has become part a long-running feud between the two activist investors that led to a shouting match on live television last month. The two Wall Street titans traded barbs on CNBC about an old investment deal that ultimately led to a fight over Ackman's position on Herbalife.
Hedge fund manager Ackman said in December that he believes Herbalife is pyramid scheme and he took a short position in the stock. Short-sellers make money when the stock they're betting against declines.
Icahn has said publicly that it was no secret that he neither liked nor respected Ackman and didn't like how he had approached his short position in Herbalife, which he said was "getting a room full of people to bad mouth the company."
He has previously refused to comment on whether he held a position in Herbalife, but many speculated that Icahn bought a stake in a move against his old adversary. He joins another investor, Dan Loeb of Third Point LLC, who previously disclosed an 8.2 percent stake in Herbalife.
Herbalife shares have had a wild ride in the past few months, as it became a flashpoint for a number of high-profile investors. Greenlight Capital's David Einhorn, another prominent Wall Street figure, raised concerns about Herbalife's business in May. And shares dropped sharply following Ackman's comments in December.
The company has disputed Ackman's characterizations, meeting with analysts and investors in January to detail how its business operates and who its customers are. Ackman replied that Herbalife "distorted, mischaracterized, and outright ignored large portions" of his presentation.
Representatives from Icahn's firm, Icahn Capital Management and Ackman's firm, Pershing Square Capital Management, did not immediately comment on Icahn's Herbalife stake.
In an emailed response, a spokeswoman for Herbalife said, "We welcome all parties who see the same value in Herbalife that we do."
Herbalife soared $8.18, or 21.4 percent to $46.22 in Thursday after-hours trading following news of Icahn's holdings.