Rainbow trout gill filaments generated a wide range of eicosanoid products following calcium ionophore challenge. The putative lipoxygenase products were separated by reverse phase high performance liquid chromatography (RP-HPLC), while prostanoids were quantified by enzyme immunoassay. Three main monohydroxy compounds containing conjugated dienes were observed after RP-HPLC namely 12-(S) hydroxyeicosatetraenoic acid (12-HETE), 12-(S) hydroxyeicosapentaenoic acid (12-HEPE) and 14-(S) hydroxydocosahexaenoic acid (14-HDHE), derived from endogenous arachidonic, eicosapentaenoic and docosahexaenoic acids, respectively. Their identification was confirmed by mass spectrometry. A further five compounds containing conjugated trienes were also observed but in lesser amounts. One of these products was identified as 8,15-dihydroxyeicosatetraenoic acid (8,15-DiHETE) based on its UV spectrum, co-elution with authentic standard on RP-HPLC and mass spectrometry. Overall, the generation of these products suggests the presence of 12- and possibly 15-lipoxygenase activities in trout gill acting on endogenous sources of fatty acid. To determine if the various cell types in trout gill had differing eicosanoid generating potential, gills were disrupted and the resultant cell suspensions separated by density gradient centrifugation. Following this three bands were formed on the gradients and the cell populations from these were characterised using periodic acid Schiff's (PAS) reactivity for mucosubstances, haematoxylin and eosin staining, and immunoreactivity with both monoclonal and polyclonal antibodies. The first band consisted of polygonal cells and other more minor cell types, the second cell band contained mainly polygonal and PAS-positive goblet epithelial cells, while the third band consisted of mainly erythrocytes. There were significant differences in the eicosanoid generating potential of the isolated cells, with cells from the second band generating significantly more 12-HETE and 8,15-DiHETE than those from both the first band and unfractionated populations. The eicosanoid generating activity of the trout gill epithelial cell line, RTG-W1, was also elucidated. It proved to be a modest generator of eicosanoids in that only low levels of thromboxane B2 and prostaglandin E2 were detected while no lipoxygenase products were observed.