The rainbow trout interleukin (IL)-1 beta gene consists of six exons/five introns, in contrast to mammals which have seven exons/six introns. The missing exon appears to be at the 5' end of the gene, probably equivalent to exon 1 or 2 of mammals. Fewer and smaller introns make the trout IL-1 beta only half the size of mammalian IL-1 beta genes. Highest homology (> 60% amino acid similarity) is seen between exon 5 of trout and exon 6 of mammals. Reverse transcription-PCR analysis revealed two additional incompletely spliced variants of the trout IL-1 beta gene, containing intron 5 or introns 4 and 5. All three transcripts are detectable in gill, kidney, liver and spleen tissue from bacterially challenged fish but only the fully spliced transcript is detectable in blood. Northern blot analysis revealed that the latter transcript is also induced following stimulation of kidney leucocytes with lipopolysaccharide for 4 h. A second inducible transcript is also detected but is larger (approximately 3 kb) than any of the above, suggesting that it could be from a second gene. Southern blot analysis also suggests at least two copies of the IL-1 beta gene or genes related to the 3' end of the IL-1 beta sequence, are present in trout.