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Files with the extension '.wk1' at the end of their name are written in the Lotus 123 format. If you would like to view files that have a .wk1 extension, you will need to use a spreadsheet program. If you are a Windows user (3.1, 95/98, NT/2000/XP), you can download the KeyView program that IBM provides for viewing .wk1 files. With it, you can view (but not edit) Lotus 123 files. There is no Lotus viewer for Mac.
If you are using a stand-alone computer that does not have Microsoft Excel installed, the Excel 97/2000 Viewer allows you to open and view Excel 97 and Excel 2000 spreadsheet (.xls) files. You can download the Excel Viewer 2003 from here.