Microsoft Corp has put its estimated $1 billion media buying account into review, raising the possibility the business could move to an agency other than current partner Universal McCann, according to a source familiar with the account.

Universal McCann is a division of advertising holding company Interpublic Group and is a major player on Madison Avenue, with blue-chip clients including Exxon Mobil, Johnson & Johnson, and Sony Corp.

It would be highly unusual if Universal McCann chose not to defend the account during Microsoft's review. Given its size and prestige, the account would likely attract keen interest from competing agencies.

As a business best practice, Microsoft regularly evaluates its agency mix to ensure we retain the most qualified partners for our business needs across all markets, Microsoft said in a statement.

The review was first reported by Advertising Age, which said Microsoft ranks No. 32 on its list of top U.S. advertisers, with a domestic advertising budget alone totaling more than $1.1 billion.

Universal McCann was not immediately available for comment.

Shares of Universal McCann parent Interpublic were up 39 cents at $11 on the New York Stock Exchange. Microsoft shares were down 53 cents at $28.30 on the Nasdaq.

(Reporting by Paul Thomasch and Bill Rigby; Editing by Phil Berlowitz)