The MEROPS database (http://www.bi.bbsrc.ac.uk/Merops/Merops.+ ++htm) provides a catalogue and structure-based classification of peptidases (i.e. all proteolytic enzymes). This is a large group of proteins (approximately 2% of all gene products) that is of particular importance in medicine and biotechnology. An index of the peptidases by name or synonym gives access to a set of files termed PepCards each of which provides information on a single peptidase. Each card file contains information on classification and nomenclature, and hypertext links to the relevant entries in online databases for human genetics, protein and nucleic acid sequence data and tertiary structure. Another index provides access to the PepCards by organism name so that the user can retrieve all known peptidases from a particular species. The peptidases are classified into families on the basis of statistically significant similarities between the protein sequences in the part termed the 'peptidase unit' that is most directly responsible for activity. Families that are thought to have common evolutionary origins and are known or expected to have similar tertiary folds are grouped into clans. The MEROPS database provides sets of files called FamCards and ClanCards describing the individual families and clans. Each FamCard document provides links to other databases for sequence motifs and secondary and tertiary structures, and shows the distribution of the family across the major kingdoms of living creatures. Release 3.03 of MEROPS contains 758 peptidases, 153 families and 22 clans. We suggest that the MEROPS database provides a model for a way in which a system of classification for a functional group of proteins can be developed and used as an organizational framework around which to assemble a variety of related information.