In this study the rainbow trout (Oncorhynchus mykiss) interleukin-2 (IL-2) cDNA has been cloned, and its expression and bioactivity analysed in head kidney leucocytes. The IL-2 precursor encoded an open reading frame of 429 bp, that translates into a predicted protein of 142 aa, with a 20 aa signal peptide. The trout IL-2 had moderate protein homology (30.9% identity/48.3% similarity) with Fugu IL-2, the only IL-2 homologue identified in fish to date, with lower homology to avian (17.8% identity/23.2% similarity) and mammalian (34.2 identity/46.5% similarity) IL-2s. IL-2 expression was induced by the T cell mitogen PHA and by the mixed leucocyte reaction, where leucocytes from pairs of fish were cultured together for four days. Expression was also induced in vivo during bacterial (Yersinia ruckeri) infection. The Escherichia coli produced recombinant IL-2 was shown to increase the expression of two transcription factors, STAT5 and Blimp-1, known to be involved in IL-2 signalling in mammals, as well as IFN-gamma, gIP and IL-2 itself. The potential signalling pathways involved and possible use as an adjuvant for fish vaccines are discussed.