A cDNA library was constructed from turbot, Scophthalmus maximus, spleen by unidirectional cloning. A total of 3 656 ESTs from the library were sequenced and compared with sequences in the GenBank database. 1 765 clones (48.3%) appeared to be completely unknown and are likely to represent newly described genes, whereas 1 891 clones (51.7%) were identified based on matches to sequences in the database. These identified clones were derived from at least 149 genes which were categorized into six categories: 9 in cell structure/motility (6.0%), 26 in metabolism (17.5%), 45 in cell defense/immunity (30.2%), 43 in gene/protein expression (28.9%), 16 in cell signal transduction/communication (10.7%), and 10 genes lacking enough information to be classified (6.7%). Immune related cDNAs identified from the spleen were: CC/CXC chemokine, MHC class Iα, complement component C1/C₃, natural killer cell enhancing factor (NKEF), thymosin beta4/ beta12, interferon inducible protein Gig1, ILB4 receptor, STAT4, and hot shock protein 70/90.