Convincing molecular evidence for the existence of both cytokines and their receptors in teleost fish is presented. TGF-β is present in <I>O. mykiss</I> encoding a 112 amino acid mature peptide. An integrin binding site (RGD) and a characteristic tetrabasic cleavage site (RKKR) are present, as is the TGF-β superfamily motif. The mature peptide has 9 conserved cysteine residues (8 of which occur in pairs) as well as two additional conserved TGF-β superfamily residues (Pro₃₆ and Gly₄₆). TGF-β exhibits a wide range tissue distribution including head-kidney macrophages, PBL, brain, gill and spleen tissue, and is encoded by a 2.5Kb mRNA. The trout TGF-β gene is spread over 7 exons, with an additional intron in exon 7 when compared to mammalian and avian models. Isolation of a partial sequence also reveals the presence of TGF-β in a cyprinid species. Phylogenetic analysis suggests trout TGF-β to cluster with mammalian TGF-β1 isoforms, and the avian (TGF-β4) and amphibian (TGF-β5) homologs. Neither TNF-α or TNF receptors were detected in <I>O. mykiss</I> at either the cellular or molecular level. The use of degenerate primers in PCR lead to the isolation of a partial sequence for <I>O. mykiss</I> MHC class I. A full length CXC-R gene of 1.6Kb isolated from <I>O. mykiss</I> displays approximately 65% identity to mammalian CXC-R4 receptors, exhibits the seven-transmembrane domain structure of the G-protein coupled receptors and a tissue-specific distribution. Characteristic superfamily motifs and a putative glycosylation site are present in the sequence. Along with the major features of the adaptive immune response such as the major histocompatibility complex (MHC), T-cell receptor (TCR) and immunoglobulin (Ig), cytokines are now shown to be present at the level of teleost fish.