This investigation provides the first conclusive evidence for the existence of the interleukin 2 (IL-2) and IL-21 genes in bony fish. The IL-2 and IL-21 sequences have been determined in Fugu rubripes by exploiting the conservation of synteny that is found between regions of the human and Fugu genomes. The predicted 149-amino acid IL-2 homologue contains the IL-2 family signature, has a predicted secondary structure of three alpha helixes and has the two cysteines important in disulphide-bond formation. It shows low amino acid identities (24-34%) with other known IL-2 sequences. The predicted 155-amino acid IL-21 homologue has a predicted secondary structure of four alpha helixes and has the four cysteines important in disulphide-bond formation. It shows low amino acid identities (29-31%) with other known IL-21 sequences. The gene organisation of Fugu IL-2 and IL-21 and the level of synteny between the human and Fugu genomes has been well conserved during evolution, with the order and orientation of the genes matching exactly to human Chromosome 4. Phytohaemagglutinin stimulation of Fugu kidney cells resulted in a large increase in the Fugu IL-2 and IL-21 transcripts. In vivo stimulation of Fugu with LPS and poly I:C showed IL-21 expression to be localised within mucosal tissues. The discovery of IL-2 and IL-21 in fish will now allow more detailed investigations into T-helper cell responses.