(Berkshire Hathaway story updated for closing price and volume spike)

OMAHA, Neb. ( TheStreet) -- The market may have been expecting Berkshire Hathaway's ( BRK.B) addition to the S&P 500 for some time already, but it didn't act that way on Wednesday.

Just under 20 million Berkshire Hathaway B shares were traded on Wednesday. That would equate to well over a year of trading in Berkshire Hathaway's B shares at its previous average daily volume of 41,000 shares.

Even though Berkshire Hathaway has had some big trading days in the past week -- on Jan. 21, close to 15 million shares were traded -- Wednesday was the biggest day yet for trading in Warren Buffett's stock.

Of course, the recent big days of trading in Berkshire Hathaway were preceded by the 50-to-1 stock split in Berkshire's B shares, which created a level of liquidity in the shares that finally allowed S&P to invite Warren Buffett into the biggest market benchmark.

What's more, Berkshire Hathaway shares were up close to 5% on a day when all the broad market indexes were up by much smaller gains, after the Fed said it would keep interest rates near current historically low levels. The financial sector to which Berkshire is most closely linked was up 1% at the close on Wednesday.

This week's hubbub surrounding the Berkshire Hathaway B shares started on Tuesday night after S&P announced that Berkshire Hathaway would replace its newest acquisition Burlington Northern ( BNI) in the market bellwether S&P 500 Index.

In the after-market session on Tuesday and pre-market session on Wednesday, Berkshire was up more than 8%.

Berkshire Hathaway's gain of just under 5% by the close on Wednesday also easily beat the S&P 500's rise of 0.5% on Wednesday. Take that S&P, from your newest holding. Maybe now the all the talk about Berkshire's underperformance versus the S&P 500 in 2009 will draw to a close.

Berkshire's B shares were up 4.9%, or a little over $3.36, to a price $71.36 at the close on Wednesday.

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