Billionaire investor Carl Icahn bought 14 million shares of Herbalife for a 12.98 percent stake, according to a 13D filing with the Securities and Exchange Commission.

News of Icahn's big bet comes almost three weeks after he and activist investor Bill Ackman engaged in verbal fisticuffs on air.

(Read More: Ackman: Herbalife Short Unaffected by Icahn Stake)

Ackman had previously revealed a short interest in Herbalife, calling it "a pyramid scheme."

In the 13D filing, Icahn raised the possibility of seeking alternatives for Herbalife, including the options of recapitalization or taking the company private.

(Read More: Ackman, Icahn Hurl Accusations, Insults)

Herbalife shares closed up more than 5 percent at $38.27 per share and climbing nearly 19 percent in after-hours trading.

With his newly revealed long position, Icahn joins the likes of Third Point's Dan Loeb, who previously took an 8 percent stake in Herbalife.

Icahn is scheduled to appear on the "Fast Money Halftime Report" on Friday.

On "Fast Money," Ken Squire of 13D Monitor said that he wasn't jumping into the trade just yet.

"I'm not sure whether I'm going to take a position or not," he said. "The fact that Bill's on the other side of it is obviously meaningful to me, so we'll see what the 13D says.

"This is like the Super Bowl of activists. It's like how can you pick one vs. the other?"

Trader disclosure: On Feb. 14, 2013, the following stocks and commodities mentioned or intended to be mentioned on CNBC's "Fast Money" were owned by the "Fast Money" traders: Dan Nathan is long BBRY MARCH APR 14 PUT SPREAD; Dan Nathan is short QQQ; Dan Nathan is short SPY; Dan Nathan is long SPY Feb 1219 puts; Karen Finerman is long AAPL; Karen Finerman is long BAC; Karen Finerman is long C; Karen Finerman is long JPM; Karen Finerman is long WMT; Karen Finerman is long TGT; Karen Finerman is long MSFT; Karen Finerman is long GOOG; Karen Finerman is long WLP; Karen Finerman is short SPY; Karen Finerman is short IWM; Steve Grasso is long AAPL; Steve Grasso is long ASTM; Steve Grasso is long BA; Steve Grasso is long GOOG; Steve Grasso is long GDX; Steve Grasso is long LF; Steve Grasso is long LNG; Steve Grasso is long MHY; Steve Grasso is long P; Steve Grasso is long PXD; Steve Grasso is long NVIV; Steve Grasso is long PFE; Steve Grasso is long S; Steve Grasso is long VLO.