Berkshire Hathaway recently filed its second-quarter 13F form, revealing its stock holdings at the end of June.

Berkshire Hathaway’s portfolio added six new stocks, including the three new secret stocks. Warren Buffett was a net seller of stocks for the eleventh straight quarter.

Warren Buffett's Berkshire Hathaway built a $1.6 billion stake in UnitedHealth last quarter, marking one of its last wagers with the investor as CEO.

Quarterly filed Form 13Fs provide a way for investors to track the buying and selling activity of Wall Street's top money managers. Warren Buffett is occasionally granted the "confidential treatment" tag,

Key Takeaways Buffett has long been a big proponent of the index funds that Bogle helped bring to the masses.However, his public declaration that Bogle is an American hero may have also been influenced by the fact that one of Bogle's funds just helped Buffett win a $1 million bet.

In May, a 13F filing revealed that Berkshire Hathaway was buying mystery stocks. Now, at last, we know what they are.

Buffett owned UnitedHealth shares before selling his stake in 2010, though his Berkshire Hathaway recently acquired shares.

Berkshire Hathaway is expected to reveal its mystery stock investment in its 13-F filing on Thursday. Here’s what to expect from the conglomerate on Aug. 14.