PubChem contains the chemical structures of small organic molecules and information on their biological activities. PubChem is intended to support the Molecular Libraries and Imaging component of the NIH Roadmap Initiative. PubChem's chemical structure database may be searched on the basis of descriptive terms, chemical properties, and structural similarity. When possible, PubChem's chemical structure records are linked to other NCBI databases. These include the PubMed scientific literature database, for example, and NCBI's protein 3D structure database. PubChem also contains the results of high-throughput biological screening experiments. PubChem is organized as three linked databases within the Entrez/PubMed information retrieval system. These are PubChem Substance, PubChem Compound, and PubChem BioAssay. PubChem also provides PubChem Structure Search, a fast chemical structure similarity search tool that links to the PubChem Compound and PubChem Substance databases. More information about using each component database may be found by links above. A PubChem FTP site is also available.