An enriched classification system for cellular functions of gene products of Escherichia coli K-12 was developed based on the initial classification by Riley. In the new classification scheme, MultiFun, cellular functions are divided into 10 major categories: Metabolism, Information Transfer, Regulation, Transport, Cell Processes, Cell Structure, Location, Extra-chromosomal Origin, DNA Site, and Cryptic Gene. These major categories are further sub-divided into a hierarchical scheme. Two thousand nine hundred twenty-two gene products of E. coli K-12 were assigned to one or more functions depending on the role they play in the cell. Functional assignments were made to 66% of E. coli gene products, ranging from 1 to 16 assignments per gene product. The expansion of cellular function categories and the assignment to more than one category (multifunction) provides a more complete description of the gene products and their roles and hence better reflects the functional complexity of organisms. We believe this classification system will be useful in the field of genome analysis, both for annotation purposes and for comparative studies. The functional classification scheme and the cellular function assignments made to E. coli gene products can be accessed from the web at the databases GenProtEC (http://genprotec.mbl.edu) and EcoCyc (http://www.ecocyc.org).