A global interaction map that describes domain-domain and protein-protein interaction information for known Protein Data Bank structures. PSIbase calculates the Euclidean distance to determine interactions between possible pairs of structural domains in proteins. It is a database and file server for protein structural interaction information calculated by the PSIMAP algorithm. PSIbase also provides an easy-to-use protein domain assignment module, interaction navigation and visual tools. Users can retrieve possible interaction partners of their proteins of interests if a significant homology assignment is made with their query sequences.