Bill Ackman has two undisclosed positions, and one of them is massive

Hedge fund titan Bill Ackman, the CEO of $20 billion Pershing Square Capital, says that his fund has two new investments that they have yet to disclose.

During a conference call with Pershing Square Holdings investors on Monday morning, the hedge fund billionaire said he will have a "little more update" on the investments at the end of May. One investment is small and the other is large, he added.

Ackman said that the large one is "approaching 15% of capital." (That would be about $3 billion, according to our calculations).

Ackman added that it could be "a couple of months before we are required to make a disclosure." (Hedge funds are required to file a disclosure with the Securities and Exchange Commission if they own 5% or more of a company's stake.)

During the Q&A portion of the call, Ackman declined to say whether the investment was active or passive.

Ackman is known for typically being a long-only activist investor, taking large positions in a handful of companies and advocating for change from management to unlock more value. His most famous position right now might be his short bet against Herbalife - a multilevel marketing company that sells weight loss shakes that he believes operates a "pyramid scheme" and will go to $0.

During the first quarter of 2015, Pershing Square was up 3.4-3.5%, he said. Year-to-date, the fund is up 6 to 6.3%. Last year, the 49-year-old activist investor had big year, netting 40%, while the average hedge fund returned just 3.78% compared with the S&P 500's 13% rise.