"After 18 months of due diligence, we have concluded that it is a certainty that Herbalife is a pyramid scheme," Ackman said in an emailed statement. "Our goal was to shine a spotlight on Herbalife. To the extent that Mr. Icahn is helping achieve this objective, we welcome his involvement."
The feud between Icahn and Ackman began a decade ago over a real estate company deal that ended up in court. Icahn was forced to pay $4.5 billion to Ackman's firm, and he has never forgotten about it.
"Ackman and I have had a dispute, we've aired it. I don't want to get back into it." Icahn told CNBC.
Icahn says that he neither likes nor respects Ackman and didn't like how he had approached his short position in Herbalife. He has previously refused to comment on whether he held a position in Herbalife. Many have speculated that he would take a stake in the company to strike at Ackman.
Where does Herbalife stand in all of this? "We welcome all parties who see the same value in Herbalife that we do," a company spokeswoman said in an emailed statement.
Herbalife shares have been extremely volatile over the last year. Greenlight Capital's David Einhorn, another prominent Wall Street figure, raised concerns about the company's business in May. The stock dropped sharply after Ackman made his negative case about the company in a presentation in December.
The company has disputed Ackman's statements, saying that he is attacking it solely to benefit his short position in the company's stock. Herbalife executives called a 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.