There are increasing evidence of the potential role of antimicrobial peptides in the regulation of immune responses in mammalian species. However, the effects of these peptides in fish have yet to be investigated. In this study, we examined the transcriptional expression profile of representative immune-relevant genes in a trout macrophage cell line, RTS11, in response to three linear cationic ?-helical antimicrobial peptides (insect cecropin B, fish pleurocidin and a cecropin analogue CF17). The expression levels of two pro-inflammatory genes, interleukin-1? (IL-1?) and cyclo-oxygenase-2 (COX-2), increased in the peptide-treated RTS11 cells. The peptides did not appear to affect the expression levels of representative genes associated with antigen presentation, interferon response or JAK/STAT signal transduction. Furthermore, the induction of IL-1? and COX-2 in RTS11 by lipopolysaccharide was not adversely affected by these three antimicrobial peptides. Overall, the data indicate a pro-inflammatory effect of the three cationic antimicrobial peptides in the inflammatory response of salmonid species, suggesting a potential application of these peptides as immune adjuvant for fish vaccination.