The Plant Proteomics Database (PPDB; http://ppdb.tc.cornell.edu), launched in 2004, provides an integrated resource for experimentally identified proteins in Arabidopsis and maize (Zea mays). Internal BLAST alignments link maize and Arabidopsis information. Experimental identification is based on in-house mass spectrometry (MS) of cell type-specific proteomes (maize), or specific subcellular proteomes (e.g. chloroplasts, thylakoids, nucleoids) and total leaf proteome samples (maize and Arabidopsis). So far more than 5000 accessions both in maize and Arabidopsis have been identified. In addition, more than 80 published Arabidopsis proteome datasets from subcellular compartments or organs are stored in PPDB and linked to each locus. Using MS-derived information and literature, more than 1500 Arabidopsis proteins have a manually assigned subcellular location, with a strong emphasis on plastid proteins. Additional new features of PPDB include searchable posttranslational modifications and searchable experimental proteotypic peptides and spectral count information for each identified accession based on in-house experiments. Various search methods are provided to extract more than 40 data types for each accession and to extract accessions for different functional categories or curated subcellular localizations. Protein report pages for each accession provide comprehensive overviews, including predicted protein properties, with hyperlinks to the most relevant databases.