Yahoo and Microsoft took a big step toward uniting their instant messaging communities with a public test that began last week.

IM users with the latest versions of Windows Live Messenger and Yahoo Messenger now can send messages to people on the other network. The test is to ensure that the networks can handle the combined user base of 350 million accounts worldwide. The test service is available to people in 19 countries, including the United States.

The Yahoo-Microsoft partnership, planned since last year, creates a strong competitor to AOL's No. 1 ranked IM service: Combined, Yahoo and Microsoft will have more users in the United States. AOL has 47 million U.S. users, according to a report by research company Nielsen/NetRatings. Microsoft's MSN Messenger and Windows Live Messenger combined rank No. 2 with 28 million users, followed by Yahoo with 22 million users.

Delivering a larger, combined user base, says Laura Martin, an analyst at Soleil-Fulcrum Research, should give Yahoo and Microsoft more clout in attracting online advertising dollars.