We present an exchange specification for data describing the three-dimensional structure of biological macromolecules. The specification was designed for MMDB, a Molecular Modeling Database supported by the National Center for Biotechnology Information (NCBI), based on information from the Protein Data Bank (PDB). In the MMDB specification, the chemical structures of molecules are described hierarchically as connectivity graphs, to directly support comparison by subgraph isomorphism or assignment algorithms. Three-dimensional coordinates are linked unambiguously to nodes in the chemical graph, so that homology-derived structures may be generated directly from alignment of chemically similar groups. In conversion to this form, data from PDB are extensively validated, so as to provide a description of chemical and spatial structure that is as accurate as possible. These changes in format and content of the known structure data are intended to support development of intelligent molecular modeling applications that make use of this invaluable information resource.