One of Ackman's recent big bets is on Herbalife.
In December, the hedge fund manager said that the nutritional supplements distributor was a pyramid scheme and that he was taking a short position in the stock. Short-sellers make money when the stock they're betting against declines.
In an interview with Bloomberg Television Thursday, Icahn had said it was no secret that he neither liked nor respected Ackman and he didn't like how he had approached his short position in Herbalife, "getting a room full of people to bad mouth the company."
Icahn refused to comment on whether he held a position in Herbalife, but speculation is rife that he bought a stake in a bet against his old adversary.
Despite the ongoing fight with his rival, Icahn has reason to be cheerful this week.
Less than five months ago, the investor decided to go against the grain and make a big investment in the Internet video service Netflix, at a time when most investors were spurning the company.
Icahn bought a stake of almost 10 percent in Netflix for $324 million. The value of that stake has already almost tripled to $941 million after Netflix's stock surged this week, when the company reported an unexpected profit and an influx of 2 million U.S. subscribers to its service.
AP Business Writer Michelle Chapman contributed to this story.