Herbalife Ltd. (NYSE:HLF) shares skyrocketed 20 percent in pre-market trading on Friday after activist investor Carl Icahn revealed in his latest regulatory filing that he bought a 13 percent stake in the company, which amounted to more than 14 million shares of the company’s stock. Herbalife came into the spotlight in December when another activist investor, Bill Ackman, disclosed that he was shorting the stock and called the company the world’s “best-managed pyramid scheme.”

Herbalife

Shares of Herbalife Ltd. (NYSE:HLF) have been especially volatile in recent weeks as Ackman and other investors took sides and dueled over the company and whether it would be a good investment.

Icahn said in Thursday’s filing that he will discuss strategy with Herbalife Ltd. (NYSE:HLF) and possible talk to the company’s executives about taking it private or recapitalizing. Icahn said he believes the company is undervalued and having a “legitimate business model.”

He also said Ackman did not act appropriately when he announced his short on shares of Herbalife Ltd. (NYSE:HLF). Icahn and Ackman were on opposite sides of another major case involving shares of Hallwood Realty, which Ackman sold to Icahn. Icahn later refused to split the profits he made on those shares. The two investors have spent over seven years arguing in court over the public disagreement.

Icahn’s recent filing with the Securities and Exchange Commission indicates that he bought approximately 1.6 million shares of Herbalife Ltd. (NYSE:HLF) between Dec. 20 and Dec. 24, just days after Ackman’s Dec. 19 declaration about his massive short of the company’s stock. Icahn then purchased more shares of the company on Jan. 28 and 29. His most recent purchases of his stake in the company were done this week..

Bloomberg reports that Icahn’s stake is made up of 2.47 million shares and options on 11.54 million shares.