To study the biological activity of interleukin-$1eta$(IL-$1eta$), a proinflammatory cytokine, in nile tilapia, Oreochromis niliticus, the recombinant tilapia IL-$1eta$ was produced in E. coli cells based on pQE vector. Ni-NTA (nitriloacetic acid) metal affinity chromatography was used to purify recombinant protein. The eluted fractions exhibited a single band of protein with a molecular weight of about 25kDa, which is in close agreement with 25.4 kDa predicted by the cDNA sequence. The biological activity of the purified recombinant tilapia IL-$1eta$ was tested through its effects on IL-$1eta$ gene expression, which are known as IL-$1eta$ inducible genes in mammals and fishes. IL-$1eta$ gene expression induced by poly I:C, a synthetic double stranded RNA, was also assessed in tilapia head kidney cells. IL-$1eta$ gene expression was analysed using QPCR (quantitative polymerase chain reaction). The ratio of the indicated gene expression was expressed as the relative mRNA level to $eta$-actin mRNA level, which is constitutively expressed in macrophages. Consequently, head kidney cells incubated for three hours with rIL-$1eta$(10, 2, 1 $mu{g}$/ml) showed a dose dependent increase in IL-$1eta$ mRNA levels and 1 $mu{g}$/ml of poly I:C was also able to induce IL-$1eta$ gene expression in head kidney in tilapia.