Protein Topology Graph Library (PTGL) is a database application for the representation and retrieval of protein topologies. Protein topologies are based on a graph-theoretical protein model at secondary structure level. Different views on protein topology are given by four linear notations for their characterization. Protein topologies can be derived at different description levels considering alpha- and beta-structures. The on-line search tool is based on an object-relational database and provides a query browser for data interrogation by string patterns, keyword queries and sequence similarity. Protein topologies are represented both as schematic diagrams and as three-dimensional images.