Warren Buffett's Berkshire Hathaway Inc. (BRKA, BRKB) revealed new investments in Viacom Inc. (VIA) and General Motors Co. (GM), though both holdings were small enough to suggest they may not have been made by the company's famous chairman.

Berkshire held 10 million shares of General Motors, the once-struggling automaker, at the end of March, according to a regulatory filing released Tuesday. The stake was worth $257 million.

Its holding of 1.59 million shares of Viacom, the owner of Paramount studios and various cable networks, was valued at $75.5 million at March 31.

The holdings were the only new additions to Berkshire's portfolio in a quarter that saw the company alter or eliminate positions in several other stocks. But the size of the holdings suggested they were investments made by Berkshire's new portfolio managers, Todd Combs and Ted Weschler.

But not every move in the portfolio was revealed in the securities filing. Berkshire said it had been granted permission by the U.S. Securities and Exchange Commission to keep some information confidential for now. The last time Berkshire was given the chance to keep a new investment hidden for a time, Buffett himself was building a massive new position in International Business Machines Corp. (IBM) last year.

Among the stock picks that are commonly thought to be Buffett's, the filing revealed a larger position in long-time holding Wells Fargo & Co. (WFC), Wal-Mart Stores Inc. (WMT) and a slightly increased stake in IBM. Stakes in Kraft Foods Inc. (KFT) and Procter & Gamble Co. (PG) declined.

Berkshire also showed no stake in long-term holding Comdisco Holdings Co. (CDCO).

Buffett's company also reported additional shares of Bank of New York Mellon Corp. (BK), DirecTV Group Inc. (DTV), Liberty Media Corp.(LMCA) and dialysis provider DaVita Inc. (DVA). Stakes in Dollar General Corp. (DG), Intel Corp. (INTC) and Verisk Analytics Inc. (VRSK) dropped.

Berkshire's total U.S. stock investments were worth $75.3 billion at the end of the first quarter, up from the $66.2 billion Berkshire reported at the end of 2011. Buffett's company, like other firms that control an investment portfolio of more than $100 million, is required to report its U.S. stock holdings 45 days after the end of a given quarter.

News about Berkshire's stock picks has the power to move the market. Some money managers use the disclosure to mimic the investment success of the "Oracle of Omaha."

But Buffett, 81, has repeatedly warned that not all the moves in the investment portfolio are his. A portion of the portfolio was long managed by Lou Simpson, the investment manager at Berkshire-owned car insurer Geico Corp. He stepped down a year ago and Combs and Weschler have stepped in.

"When our quarterly filings report relatively small holdings, these are not likely to be buys I made but rather holdings denoting purchases by Todd or Ted," Buffett wrote in his widely read shareholder letter in February.

Each of the two men has been handed "a few billion dollars" of Berkshire's money to invest initially, Buffett said in the letter. But they will gradually get more of Berkshire's money to invest over time and may be joined by a third investment manager. Upon Buffett's eventual departure from the company, the managers would take over Berkshire's entire portfolio.

"They have the brains, judgment and character" for the job, Buffett wrote.

-By Erik Holm, Dow Jones Newswires; 212-416-2892; erik.holm@dowjones.com

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