If the deal goes through, analysts expect scrutiny from Congress, Justice and other enforcement agencies, but they say any concerns about search engine or online advertising market power may not be significant enough to stop the transaction.
"The antitrust division would be interested in looking at the competitive effects of the transaction," said Justice Department spokeswoman Gina Talamona.
A spokesman from the Federal Trade Commission, which in December approved Google Inc.'s $3.1 billion purchase of online advertising company DoubleClick Inc., declined to comment. That deal still faces antitrust scrutiny in Europe.
Keith Hylton, a professor of antitrust law at Boston University, said Google's success in online search and advertising means a combined Microsoft-Yahoo would have significant competition.
"The fact that Google dominates this business will be a big factor in their (Microsoft's) favor in trying to get this approved by the regulators," Hylton said.
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