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

During a conference call with Pershing Square Holdings investors on Monday morning, the hedge fund billionaire said he would 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 the large one was "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 stock.)

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 changes from management to unlock more value. His best-known position today may be his short bet against Herbalife — a multilevel marketing company that sells weight-loss shakes that he believes operates as a "pyramid scheme" and will go to $0.

During the first quarter of 2015, Pershing Square was up 3.4% to 3.5%, he said. Year-to-date, the fund is up 6% to 6.3%. The 49-year-old activist investor had a big year in 2014, netting 40% while the average hedge fund returned just 3.78% and the S&P 500 rose 13%.

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