Berkshire Hathaway (NYSE:BRK-A) (NYSE:BRK-B) filed its first-quarter 13-F on Tuesday evening, which provides details about the company's stock portfolio as of March 31, 2018. And while we already knew about Berkshire's two most significant stock transactions, there were a few other surprises for investors.

Here's what Berkshire Hathaway has been buying

Company (Symbol)

Shares Owned 12/31/2017

Shares Owned 3/31/2018

Change

% Change

Apple (NASDAQ:AAPL)

165,333,962

239,567,633

74,233,671

44.9%

Teva Pharmaceuticals (NYSE:TEVA)

18,875,721

40,539,710

21,663,989

115%

Monsanto (NYSE:MON)

11,708,747

18,970,134

7,261,387

62%

U.S. Bancorp (NYSE:USB)

87,058,877

90,847,721

3,788,844

4.4%

Bank of New York Mellon (NYSE:BK)

60,818,783

62,191,448

1,372,665

2.3%

Delta Air Lines (NYSE:DAL)

53,110,395

53,599,357

488,962

0.9%

USG (NYSE:USG)

39,002,016

43,387,980

4,385,964

11.2%

Data source: Berkshire Hathaway SEC Filings. Only positions of $200 million or more (at either quarter's end) are listed.

Obviously, Apple is the big story of the quarter, and it's one we already knew about. Buffett revealed Berkshire's increased stake in the iPhone maker a few weeks prior to this SEC filing. In fact, in a recent CNBC interview, Buffett's comments sounded like he may buy even more.

Warren Buffet speaking with the media.

Image source: The Motley Fool.

The two most significant buys that we didn't previously know about were generic drug company Teva Pharmaceuticals and Monsanto.

Teva was a new addition to Berkshire's portfolio during the fourth quarter, so it certainly makes sense that the purchases could have spanned two quarters. As my colleague Sean Williams wrote shortly after the initial stake was revealed, Teva has several "Buffett stock" qualities, such as a recession-resistant business model, strong cash flow, and a dirt-cheap valuation.

In Monsanto's case, Bayer's $128 per share takeover plan has been approved, so Berkshire stands to make about $3 per share from the current price on this investment once the deal goes through. It's worth noting that this will be an all-cash deal, so it will add about $2.4 billion to Berkshire's already massive cash stockpile.

Here's what Berkshire Hathaway has been selling

Company (Symbol)

Shares Owned 12/31/2017

Shares Owned 3/31/2018

Change

% Change

International Business Machines (NYSE:IBM)

2,048,045

0

(2,048,045)

(100%)

Phillips 66 (NYSE:PSX)

80,689,892

45,689,892

(35,000,000)

(43.4%)

Wells Fargo (NYSE:WFC)

458,232,268

456,513,244

(1,719,024)

(0.4%)

Charter Communications (NASDAQ:CHTR)

8,489,391

8,222,873

(266,518)

(3.1%)

United Continental (NYSE:UAL)

28,211,563

27,705,963

(505,600)

(1.8%)

Liberty Global (NASDAQ:LBTYA)

20,180,897

18,206,408

(1,974,489)

(9.8%)

Data source: Berkshire Hathaway SEC Filings. Only positions of $200 million or more (at either quarter's end) are listed.

As I mentioned, we already knew Berkshire disposed of the last of its IBM investment during the quarter. This wouldn't have been much of a surprise anyway. Berkshire has been getting rid of IBM shares for some time now, and Buffett has made pessimistic comments on the company, saying that he's valued it lower than he originally did.

We also knew that Buffett was selling exactly 35 million shares of Berkshire's Phillips 66 stock back to the company. This move is designed to keep Berkshire's ownership below 10% of the oil refiner in order to avoid additional regulatory scrutiny. At the time the buyback was announced in February, Buffett emphasized that Berkshire plans to keep its Phillips 66 investment long term, and that the sale was strictly for regulatory purposes.

On that note, Berkshire shareholders have wondered whether Buffett would start to unwind the massive Wells Fargo investment in the wake of the bank's scandals that have come to light over the past couple of years. Buffett's recent comments indicate that he has no plans to sell, and the only reason Wells Fargo appears in the "stocks Buffett is selling" chart is for the same regulatory reason -- to keep its ownership stake below 10% of the bank.

Should you do the same?

To be clear, we don't know if Warren Buffett made most of these decisions himself (aside from Apple and IBM), or if one of his stock-pickers made the calls.

Furthermore, while keeping track of what Warren Buffett does can be a great source of trading ideas, I never advise buying or selling any stock simply because a billionaire did -- even if that billionaire is the Oracle of Omaha.

Matthew Frankel owns shares of Apple and Berkshire Hathaway (B shares). The Motley Fool owns shares of and recommends Apple and Berkshire Hathaway (B shares). The Motley Fool is short shares of IBM and has the following options: long January 2020 $150 calls on Apple and short January 2020 $155 calls on Apple. The Motley Fool has a disclosure policy.