Microsoft and Yahoo will allow interoperability between their instant-messaging (IM) clients for the first time.

The companies are to offer limited public testing, enabling users of Windows Live Messenger and Yahoo Messenger with Voice to connect to each other through either service.

Microsoft already has enabled IM interoperability between Live Communications Server (LCS), its enterprise unified messaging software, and IM clients from Yahoo and AOL as well as its own Windows Live Messenger.

Business customers that have purchased LCS and Office Communicator, the client interface for LCS, can connect to all of those IM clients.

Wednesday marks the first time that users of the free Windows Live Messenger and Yahoo IM clients, can take advantage of interoperability between them, Microsoft said.

The beta will be available in the US as well as Argentina, Australia, Brazil, Canada, China, France, Germany, Hong Kong, India, Italy, Korea, Mexico, the Netherlands, Singapore, Spain, Taiwan, Turkey and the UK.

Microsoft and Yahoo plan to make this interoperability available to all users of Windows Live Messenger and Yahoo Messenger with Voice in the next few months, the companies said.

Microsoft declined to comment on whether it would soon have a similar deal to make Windows Live Messenger interoperate with AOL's consumer IM client, but said that the company "anticipates more of these relationships in the future."

Windows Live Messenger is the latest incarnation of what was formerly MSN Messenger.

Microsoft rebranded the product as part of its Windows Live online services plan and released the first version of Windows Live Messenger last month.