Ranaviruses (family Iridoviridae) are a growing threat to fish and amphibian populations worldwide. The immune response to ranavirus infection has been studied in amphibians, but little is known about the responses elicited in piscine hosts. In this study, the immune response and apoptosis induced by ranaviruses were investigated in fish epithelial cells. Epithelioma papulosum cyprini (EPC) cells were infected with four different viral isolates: epizootic haematopoietic necrosis virus (EHNV), frog virus 3 (FV3), European catfish virus (ECV) and doctor fish virus (DFV). Quantitative real-time PCR (qPCR) assays were developed to measure the mRNA expression of immune response genes during ranavirus infection. The target genes included tumour necrosis factor a (TNF-a), interleukin-1b (IL-1b), b2-microglobulin (b2M), interleukin-10 (IL-10) and transforming growth factor b (TGF-b). All ranaviruses elicited changes in immune gene expression. EHNV and FV3 caused a strong pro-inflammatory response with an increase in the expression of both IL-1b and TNF-a, whereas ECV and DFV evoked transient up-regulation of regulatory cytokine TGF-b. Additionally, all viral isolates induced increased b2M expression as well as apoptosis in the EPC cells. Our results indicate that epithelial cells can serve as an in vitro model for studying the mechanisms of immune response in the piscine host in the first stages of ranavirus infection. Fish & Shellfish Immunology. 2012, 32:1095-1105. Riikka Holopainen, Hannele Tapiovaara, Jarno Honkanen. Finnish Food Safety Authority Evira, Veterinary Virology Research Unit, Mustialankatu 3, FI-00790 Helsinki, Finland. riikka.holopainen@evira.fi