Two rainbow trout (Oncorhynchus mykiss) genes for interferon regulatory factors (IRF) 1 and 2 have been cloned and sequenced. The IRF-1 cDNA contains an open reading frame (ORF) of 996 nucleotides that translates into a 331 amino-acid putative peptide, with a 5' untranslated region (UTR) of 145bp and a 3' UTR of 481bp. The IRF-2 cDNA contains a 1035bp ORF that translates into a 344 amino-acid putative peptide, with a 5' UTR of 146bp and a 3' UTR of 925bp. In vivo, IRF-1 and IRF-2 are constitutively expressed in head kidney, gill and spleen but not liver. Both genes were induced in all the tissues examined. IRF-1 but not IRF-2 expression was significantly increased at the site of injection 1 week after DNA vaccination against viral haemorrhagic septicaemia virus. In vitro, IRF-1 and IRF-2 transcripts are present in unstimulated rainbow trout gonad cells and are up-regulated by poly I/C.