I.B.M. plans to mount its most ambitious challenge in years to Microsoft's dominance of personal computer software, by offering free programs for word processing, spreadsheets and presentations.

The company announced the desktop software, called I.B.M. Lotus Symphony, at an event yesterday in New York. The programs will be available as free downloads from the I.B.M. Web site.

I.B.M.'s Lotus-branded proprietary programs already compete with Microsoft products for e-mail, messaging and work group collaboration. But the Symphony software is a free alternative to Microsoft's mainstay Office programs - Word, Excel and PowerPoint. The Office business is huge and lucrative for Microsoft, second only to its Windows operating system as a profit maker.Its offerings are versions of open-source software developed in a consortium called OpenOffice.org. The original code traces its origins to a German company, Star Division, which Sun Microsystems bought in 1999. Sun later made the desktop software, now called StarOffice, an open-source project, in which work and code are freely shared.

I.B.M.'s engineers have been working with OpenOffice technology for some time. But last week, I.B.M. declared that it was formally joining the open-source group, had dedicated 35 full-time programmers to the project and would contribute code to the initiative.

Free office productivity software has long been available from OpenOffice.org, and the open-source alternative has not yet made much progress against Microsoft's Office. But I.B.M., analysts note, has such reach and stature with corporate customers that its endorsement could be significant.

I.B.M. executives compare this move with the push it gave Linux, the open-source operating system, into corporate data centers. In 2000, I.B.M. declared that it would forcefully back Linux with its engineers, its marketing and its dollars. The support from I.B.M. helped make Linux a mainstream technology in corporations, where it competes with Microsoft's Windows server software.

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