The Microsoft Biology Foundation (MBF) is a language-neutral bioinformatics tool kit built as an extension to the Microsoft .NET Framework. It implements a range of parsers for common bioinformatics file formats; a range of algorithms for manipulating DNA, RNA, and protein sequences; and a set of connectors to biological web services such as the U.S. National Center for Biotechnology Information’s Basic Local Alignment Search Tool. MBF is available under an open-source license, and executables, source code, demo applications, and documentation are freely downloadable.