Proinflammatory effects of bacterial lipopolysaccharide (LPS) have been assessed by analysing the induction of two inflammatory genes, $interleukin-1eta$ $(IL-1eta)$ and cyclooxygenase-2 (COX-2), in rainbow trout (Oncorhynchus mykiss) macrophage cells. Production of a metabolite of arachidonic acid by COX-2, prostaglandin $E_2;(PGE_2)$, was also analysed in macrophage cells after LPS stimulation. Northern blot analysis revealed that LPS $(5{mu}g/mL)$ significantly upregulated $IL-1eta$ (54 times) and COX-2 (40.7 times) gene expression in macrophage cells after 4 h stimulation. According to RT-PCR (Reverse Transcription Polymerase Chain Reaction) analysis, $IL-1eta$ gene induction in LPS stimulated macrophage cells was started within 1h and significantly increased thereafter until 4h. Meanwhile, COX-2 gene induction by LPS was delayed in comparison with $IL-1eta$ gene induction as a faint band was observed after 4h stimulation in head kidney macrophage cells. LPS also significantly increased $PGE_2$ production in head kidney leucocytes, presumably via activating COX-2 expression that metabolites arachidonic acid to $PGE_2$. In conclusion, it was demonstrated that LPS could induce two main inflammatory and immune related genes, $IL-1eta$ and COX-2, and increase $PGE_2$ production in trout head kidney macrophage cells, representing a strong inflammatory activity.