In situ hybridisation and immunohistochemistry analyses have shown that the Ciona intestinalis tumour necrosis factor alpha gene (CiTNFalpha), which has been previously cloned and sequenced, is expressed either during the inflammatory pharynx response to lipopolysaccharide (LPS) or during the swimming larval phase of development. Granulocytes with large granules and compartment/morula cells are CiTNFalpha-producing cells in both inflamed pharynx and larvae. Pharynx vessel endothelium also takes part in the inflammatory response. Haemocyte nodules in the vessel lumen or associated with the endothelium suggest the involvement of CiTNFalpha in recruiting lymphocyte-like cells and promoting the differentiation of inflammatory haemocytes. Specific antibodies against a CiTNFalpha peptide have identified a 43-kDa cell-bound form of the protein. Observations of pharynx histological sections (at 4 and 8 h post-LPS inoculation) from naive and medium-inoculated ascidians have confirmed the CiTNFalpha-positive tissue response. Larval histological sections and whole-mount preparations have revealed that CiTNFalpha is expressed by trunk mesenchyme, preoral lobe and tunic cells, indicating CiTNFalpha-expressing cell immigration events and an ontogenetic role.