Eosinophilic granule cells (EGCs) found in the gills, skin and alimentary canals of fish have been likened to mammalian mast cells in terms of their structure and function. To investigate this situation further, gill explant cultures from the rainbow trout, Oncorhynchus mykiss, were set-up and incubated with either lipopolysaccharide (LPS; 5 micrograms ml-1) or human recombinant tumour necrosis factor-alpha (TNF-alpha; 25 iu ml-1) alone or in combination for 7 days. Examination of histological sections of these gill explants after this incubation showed a significant increase in the number of EGCs in those explants incubated with a combination of LPS and TNF-alpha compared with the control. Similarly, exposure of trout to short-term (> 6 h) handling and confinement stress resulted in a significant increase in the number of EGCs in the gills, while longer term stress (> 6 days) was without significant effect. The EGCs in the gills were shown to contain granules that reacted with both basic dyes, such as methylene blue, and eosin but failed to react with periodic acid Schiff's reagent. Of particular interest was the finding that only some of the EGCs reacted with the leucocyte-specific monoclonal antibody, 21G6, suggesting some heterogeneity within this cell type in the gill.