BioNumbers is a database of key numbers in molecular and cell biology, providing quantitative properties of biological systems important to computational, systems, and molecular cell biologists. The database includes a wide range of values, such as cell sizes, metabolite concentrations, reaction rates, generation times, genome sizes, and the number of mitochondria in a cell. BioNumbers was created to address the challenge of finding accurate and relevant numbers in the vast biological literature, which can be time-consuming and frustrating. All numbers are directly sourced from literature and are cited with their references. The database is searchable by keywords or menus and allows registered users to contribute and comment on entries. All entries are curated to maintain high quality. BioNumbers currently contains over 4500 distinct properties from more than 200 organisms, extracted manually from over 1000 references. The database is dynamic and continuously updated by curators and the scientific community. It provides a portal to literature and online resources rich in biological numbers. BioNumbers is not a one-stop destination but the first step in the search for biological numbers. It includes links to other online tables of useful numbers and HTP repositories. The database is implemented as a Microsoft ASP.NET 2.0 web application with a Microsoft SQL Server 2005 database. The search is performed using SQL commands and integrates results with flexible weights. The database is curated by the BioNumbers team and registered users. Each entry has a unique BioNumber identification number (BNID). The database is continuously expanded and advanced, with future plans including a 'BioNumber of the month' feature and a comparative table builder. BioNumbers is an integrated and practical portal that will provide researchers with a greater ability to do biology by the numbers.BioNumbers is a database of key numbers in molecular and cell biology, providing quantitative properties of biological systems important to computational, systems, and molecular cell biologists. The database includes a wide range of values, such as cell sizes, metabolite concentrations, reaction rates, generation times, genome sizes, and the number of mitochondria in a cell. BioNumbers was created to address the challenge of finding accurate and relevant numbers in the vast biological literature, which can be time-consuming and frustrating. All numbers are directly sourced from literature and are cited with their references. The database is searchable by keywords or menus and allows registered users to contribute and comment on entries. All entries are curated to maintain high quality. BioNumbers currently contains over 4500 distinct properties from more than 200 organisms, extracted manually from over 1000 references. The database is dynamic and continuously updated by curators and the scientific community. It provides a portal to literature and online resources rich in biological numbers. BioNumbers is not a one-stop destination but the first step in the search for biological numbers. It includes links to other online tables of useful numbers and HTP repositories. The database is implemented as a Microsoft ASP.NET 2.0 web application with a Microsoft SQL Server 2005 database. The search is performed using SQL commands and integrates results with flexible weights. The database is curated by the BioNumbers team and registered users. Each entry has a unique BioNumber identification number (BNID). The database is continuously expanded and advanced, with future plans including a 'BioNumber of the month' feature and a comparative table builder. BioNumbers is an integrated and practical portal that will provide researchers with a greater ability to do biology by the numbers.