Biological activities of a putative mature sea bass interleukin-1beta peptide, produced as a recombinant protein (rIL-1beta) in Escherichia coli, have been investigated. The rIL-1beta contains a 6-histidine tag at the N-terminus, and protein purification has been achieved through this tag by affinity chromatography. Biological activities have been investigated both at the cellular and gene expression levels. In in vitro assays sea bass rIL-1beta induced the proliferation of murine D10.G4.1 cells and increased yeast phagocytosis by sea bass head kidney leukocytes. The purified cytokine was also tested in a lymphocyte-activation factor assay, where it induced the proliferation of sea bass thymocytes. Finally, in an in vivo assay, rIL-1beta administered intraperitoneally increased expression levels of the IL-1beta gene and activated macrophages to produce a cyclooxygenase 2 homologue (COX-2) gene in the head kidney.