Peptidases (proteolytic enzymes or proteases), their substrates and inhibitors are of great relevance to biology, medicine and biotechnology. The MEROPS database ( http://merops.sanger.ac.uk ) aims to fulfil the need for an integrated source of information about these. The organizational principle of the database is a hierarchical classification in which homologous sets of peptidases and protein inhibitors are grouped into protein species, which are grouped into families and in turn grouped into clans. Important additions to the database include newly written, concise text annotations for peptidase clans and the small molecule inhibitors that are outside the scope of the standard classification; displays to show peptidase specificity compiled from our collection of known substrate cleavages; tables of peptidase–inhibitor interactions; and dynamically generated alignments of representatives of each protein species at the family level. New ways to compare peptidase and inhibitor complements between any two organisms whose genomes have been completely sequenced, or between different strains or subspecies of the same organism, have been devised.