The intron-exon organisation of the carp IL-1 gene consists of 2455 bp and comprises seven exons. Three IL-1 RNA transcripts have been found in carp: (1) a fully spliced product; (2) exon 1–7 with introns 5 and 6; and (3) exon 1–7 with intron 5 only. The intron-containing products probably represent partially spliced transcripts. IL-1 mRNA expression in carp was semi-quantitatively analysed by RT-PCR in multiple organs, including brain and pituitary. Constitutive expression of the IL-1 mRNA was found in these organs with a predominant expression in the immune organs head kidney and spleen. Furthermore, a scattered distribution of IL-1 producing cells was shown by in situ hybridisations of head kidney tissue. Administration of phorbol-myristate-acetate (PMA), lipopolysaccharide (LPS) or retinoic acid (RA), to phagocytes isolated from the head kidney, resulted in expression of IL-1 intron-containing transcripts. Of these, only PMA and LPS were stimulators that induced the fully spliced transcript. A role for the nuclear factor (NF)-B pathway in carp IL-1 expression was shown with suppression of the LPS-induced IL-1 expression by NF-B inhibitor pyrrolidine dithiocarbamate (PDTC). Cortisol was able to inhibit in vitro constitutive expression of IL-1 transcripts. Addition of cortisol simultaneously with LPS could not substantially inhibit transcription.