Google is pitching advertisers on a new kind of ad targeting that aims to improve on the ubiquitous tracking cookie.

The technology allows advertisers to target people who've visited their web sites with ads on tablets and smartphones, according to agency execs who've been briefed. Google sent documents to a number of agencies to brief them on a beta test of the program within the past two weeks.

Last fall word leaked that Google and Microsoft were working on a replacement for the tracking mechanisms called cookies that companies use to track people's online behavior and target them accordingly with ads. This beta program is not that.

However, it does bridge the desktop-to-mobile gap, which has been considered a top motivation for Google's cookie-replacement plans since cookies currently don't really work on mobile devices.

"We're always running experiments with our agency partners. This small trial started last year, testing a way for marketers to reach their customers across devices using their own customer data," said a Google spokesperson.

Here's how the new retargeting tests work. Google gives an advertiser what's called a "hashed tag" to drop when someone logs into the advertiser's site. This "hashed tag" is basically an anonymized tracking identifier that ties to the cookies and device identifiers used by Google's ad-tech systems.

After the tag is dropped on someone's computer, the advertiser can show ads to that person on any property within Google's network of third-party sites and mobile apps. Brands had previously been able to retarget people through Google but were limited to web browsers or mobile apps, depending on which channel someone used to visit their property.

While Google's retargeting program incorporates advertisers' customer databases, it is not the same as Facebook's Custom Audiences targeting feature the pinpoints ads based on marketers' lists of customers' email addresses and phone numbers.

Unlike the social network's program, advertisers are not uploading lists of customers' email addresses to Google. However Google's program is similar to the spun-off version of Custom Audiences Facebook rolled out last October to retarget advertisers' customers within Facebook's desktop and mobile properties.

Google is looking for advertisers that operate sites with registered user bases totaling at least 100,000 people including a significant percentage on mobile, according to documents Google is using to pitch agencies on the test program. The test phase is limited to U.S. audiences.