Coinfection by infectious hematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV), which results in the inhibition of IHNV, has been previously characterised in salmonid fish. In the current work, we have studied whether this coinfection has repercussions in the early immune response by studying the expression of two pro-inflammatory cytokines, interleukin 1β (IL-1β) and the chemokine, IL-8 in brown trout (Salmo trutta). First, the molecules were amplified using primers previously described in rainbow trout (Oncorhynchus mykiss), and partial brown trout sequences obtained. At this point, RT-PCRs were optimised for the detection of both cytokines in brown trout experimentally infected with either IPNV, IHNV, IHNV 3B (mutant IHNV obtained from a natural coinfection with IPNV), or coinfected with an IHNV-IPNV (1 1) sample, which consisted in a mix of IHNV and IPNV reference strains, containing equal infectious titres of each virus. Differences were observed in the expression of IL1β and IL8 in response to the different viruses. The IPNV-IHNV coinfection induced a lower IL-1β expression than that expected from the results with single infections, while the mutant IHNV 3B provoked an overall significantly higher immune response. © 2006 Elsevier B.V. All rights reserved.