Google's chief legal officer Monday fired the first shot of what will likely be an antitrust salvo at Microsoft over its unsolicited $44.6 billion bid for Yahoo. Within two hours, Microsoft countered.

Saying that the offer "raises troubling questions" about Microsoft's moves and arguing that a deal might threaten the "underlying principles of the Internet: openness and innovation," David Drummond, Google's chief legal officer, explicitly laid the antitrust card on the table.

"Could Microsoft now attempt to exert the same sort of inappropriate and illegal influence over the Internet that it did with the PC?" Drummond asked in a post to Google's corporate blog on Sunday. "While the Internet rewards competitive innovation, Microsoft has frequently sought to establish proprietary monopolies -- and then leverage its dominance into new, adjacent markets."

In a statement released about 90 minutes after Drummond's post went live, Microsoft's chief counsel, Brad Smith, shot back, saying that a Microsoft-Yahoo merger would "create a more competitive marketplace," not stymie competition.

Drummond cited instant messaging (IM), Web-based e-mail and portals as areas of concern. "Microsoft plus Yahoo equals an overwhelming share of instant messaging and Web e-mail accounts. And between them, the two companies operate the two most heavily trafficked portals on the Internet," Drummond claimed. "Could a combination of the two take advantage of a PC software monopoly to unfairly limit the ability of consumers to freely access competitors' e-mail, IM, and Web-based services?"

But those number bear more objective examination. According to data from comScore Media Metrix, a combined Microsoft and Yahoo would be the No. 1 Web property. Together, the two companies in December attracted 1.02 billion visitors, nearly double Google's 588 million visitors during the same month. However, Google has the lion's share of the search market; comScore's most recent numbers peg Google's worldwide share at 62% and a combined Yahoo and Microsoft at 16%.

Google has brought up antitrust issues before when it thought that Microsoft was trespassing on its turf. In late 2006, for instance, it complained to the U.S. Department of Justice that Windows Vista's integrated desktop search "violates its agreement with the government and hurts consumers." Google argued that Vista did not let users choose an alternate search engine, presumably Google's own.

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