Google wants beta testers for a planned mobile version of Adsense.

Google's AdSense program lets website developers earn revenue by placing ads on their sites. Google runs the back-end network that places ads relevant to site content. Site owners earn revenue when visitors click on the ads.

For the Adsense for Mobile test, sites must be written in one of three mobile markup languages: (Wireless Markup Language), XHTML (Extensible Hypertext Markup Language) and CHTML (Compact HTML). That's because Google's crawlers must be able to read the page to determine page content and serve relevant ads.

A blogger at Self Made Minds said that he received an email invitation from Google last Thursday, to test AdSense for Mobile.

Google has confirmed that it is conducting a limited beta of AdSense for Mobile. The company plans to evaluate the test and refine the product based on feedback from users, it said.

Late last year, Google began delivering advertisements along with its mobile search results.

Online giants like Google are increasingly interested in the potential revenue stream from mobile users.

"Mobile advertising is a huge opportunity for us starting with the basic premise that there are something like three billion or so handsets in the world," said Dilip Venkatachari, director of product management responsible for mobile monetisation efforts at Google, in a recent interview. That compares to just around one billion PC users on the planet, making for an even larger target market.