A full length cDNA of a putative common cytokine receptor gamma chain (gammaC) gene of rainbow trout (Oncorhynchus mykiss) has been cloned and sequenced. The contiguous cDNA contained 2291 nucleotides, consisting of an ORF of 1029 bp, with a 72 bp 5' UTR and a 1190 bp 3' UTR. The coding region showed 44-46% identity to mammalian gammaC genes. The ORF translated into a 343 amino acid protein, with some 28-30% amino acid identity to the coding region of mammalian sequences. A predicted signal peptide and transmembrane domain were identified, giving a 206 amino acid extracellular domain and a 98 amino acid intracellular domain in the trout molecule. Five potential glycosylation sites were present in the extracellular domain, as were six conserved cysteine residues and the W-S-X-W-S motif typical of haemopoietin receptors. One of the most interesting differences between the trout and mammalian sequences was the lack of tyrosines in the trout intracellular domain. RT-PCR studies revealed a wide tissue distribution of gammaC expression, with detectable transcript in blood, spleen, gill, kidney, brain and liver. Low levels of gammaC transcript were detectable in unstimulated macrophage cultures and expression was increased by stimulation of the cells with recombinant trout interleukin-1beta (IL-1beta) or LPS. Similarly, in the RTG cell line which exhibited even lower level constitutive expression, stimulation with IL-1beta increased gammaC transcript levels but LPS had no effect.