Buffett, Berkshire's chairman and chief executive officer, said "one of the fellows in the office that manage money" made the purchases, a reference to investment managers Todd Combs and Ted Weschler.
Shares of Amazon hit highs of the year after Buffett discloses investment, but said he wasn't the person behind decision. Buffett has pointed to Apple's strong profitability as one of the reasons to bet on the company.
The Amazon stake further cements the relationship between Berkshire Hathaway and Amazon.
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Buffett has championed Amazon and its CEO, Jeff Bezos, in the past, lauding Amazon's dominance and Bezos' business acumen. While Berkshire Hathaway's stake in Amazon might not be a game changer, it could decrease some of the valuation concerns.
The event, second of its kind following its inauguration past year, came as US multinational conglomerate Berkshire Hathaway will be holding its annual shareholders' meeting Saturday.
Buffett has mostly avoided technology-related stocks over the years, saying he didn't understand the products and markets well enough.
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Buffett's assistants did not immediately respond to a message seeking comment.
One exception was IBM Corp. "They got into Apple late in the game-from nowhere five years ago to now it's their biggest holding", Cathy Siefert, equity research analyst at CFRA Research, told CBS MoneyWatch.
We spoke to folks at Brooks Shoes, who showed off its footwear designed for shareholders, which happen to be the same ones they gave Warren Buffett. Knowing this, a headline that Berkshire Hathaway was buying shares in Amazon would understandably generate interest in the markets.
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