May 15 (Bloomberg) -- Billionaire Warren Buffett's Berkshire Hathaway Inc. added to stakes in Kraft Foods Inc. and Wells Fargo & Co. in the first quarter, taking advantage of prices driven down by the credit crisis and slowing U.S. economy.

Buffett also increased holdings in health insurers UnitedHealth Group Inc. and WellPoint Inc., according to a regulatory filing today by Omaha, Nebraska-based Berkshire. The Standard & Poor's 500 Index declined 9.9 percent in the first three months of the year.

``If a stock goes down 50 percent it doesn't bother me in the least,'' Buffett told reporters earlier this month after Berkshire's annual shareholder meeting in Omaha. ``If we're going to be buying things, we want to buy them on sale.''

Buffett, 77, built Berkshire from a failing maker of men's suit linings into a $200 billion holding company with a $72.6 billion stock portfolio by investing premiums from insurance subsidiaries such as Geico Corp. and National Indemnity Co. Berkshire is the largest shareholder of Coca-Cola Co., Wells Fargo, Kraft and American Express Co. as of March 31, according to Bloomberg data.

Berkshire's holdings of Kraft, the world's second-biggest foodmaker, rose 4.4 percent since Dec. 31 to 138.3 million shares. Buffett first disclosed a stake in the Northfield, Illinois-based company at the end of 2007. The shares fell 5 percent in the first quarter. Berkshire's filing discloses U.S. equity investments as

SAN FRANCISCO (AP) -- Yahoo Inc. Chief Executive Jerry Yang spent months fending off Microsoft Corp.'s unsolicited takeover bid. Now he may only have a few weeks to persuade the software maker to revive its last offer of $47.5 billion, or risk being fired in a shareholder mutiny led by activist investor Carl Icahn.

Spurred on by outraged shareholders, Icahn notified Yahoo Thursday that he will lead a revolt to oust Yang and the rest of the Internet company's board unless they renew negotiations with Microsoft that fell apart May 3 when the two sides couldn't agree on a price.

In a response late Thursday, Yahoo Chairman Roy Bostock signaled that the Sunnyvale-based company is prepared to battle the New York financier.

Bostock criticized Icahn for having a "significant misunderstanding of the facts" about Microsoft's offer and the Yahoo board's response. He also emphasized that Yahoo remains open to a sale "if it offers our stockholders full and certain value."

To pressure Yahoo, Icahn has nominated an alternate slate of directors to replace the current board in an election scheduled July 3 at Yahoo's annual meeting. If the uprising is successful, an Icahn-led board presumably would fire Yang as CEO and try to negotiate a sale to Microsoft.

In his letter to Bostock, Icahn lambasted the board's actions as "irresponsible" and "unconscionable," given that Yahoo's stock stood at $19.18 before Microsoft first made its bid. He urged the board to