BOSTON ( TheStreet) -- Warren Buffett's Berkshire Hathaway ( BRK.B) made only two trades in the first quarter: initiating a small stake in credit-card company MasterCard ( MA) and trimming its big stake in oil company ConocoPhillips ( COP).It's the second quarter in a row of little activity for the investment company run by the man dubbed the Oracle of Omaha for the trading prowess that helped him build an estimated net worth of $50 billion over half a century. In the first quarter, former hedge fund manager Todd Combs started working at Berkshire, managing part of the portfolio, as the 80-year-old billionaire selects a succession team. Berkshire Hathaway's 26-stock investment portfolio was valued at $53.6 billion as of March 31, up $1 billion, or 2%, from the end of 2010, according to a Securities and Exchange Commission report that the hedge fund filed late Monday. Buffett isn't required to publish foreign holdings. The benchmark S&P 500 Index rose 5.4% in the first quarter.