Yahoo! has announced that it will test Google’s online advertising platform Adsense for a limited period on its search service. Yahoo search marketing was a major rival to Google's AdSense.

The partnership between Google and Yahoo! has come as a big shock for software major Microsoft. Microsoft had shown interest in buying Yahoo! in January this year when the stock price of Yahoo! was hovering around its 52-week low. Yahoo! declined Microsoft's takeover bid saying that the offer undervalues the company.

In an official statement released on Wednesday, Yahoo said that it will test Google's AdSense for two weeks on not more than 3% of its search queries.

Brad Smith at Microsoft believes that any agreement between Google and Yahoo! will result in Google's monopoly in the search advertising market. He further added that Microsoft is having a close watch on the whole situation and the company’s US $44.6 billion takeover offer remains valid.

If Yahoo! and Google joined hands, it will offer 90% of search advertising market share to Google. Google Inc. stock closed at $464.19 on April, 9 after a decline of $3.62 compared to previous close. Yahoo! Inc. stock closed at $27.77 with a marginally higher close over previous session.