Monoclonal antibodies were raised against head kidney macrophages of the rainbow trout, Oncorhynchus mykiss. Despite the establishment of a significant number of different hybridoma clones, none of these released antibody specific for determinants only found on macrophages. Instead, all the monoclonal antibodies generated reacted with lymphocytes, granulocytes, and monocytes/macrophages, although thrombocytes (the platelet equivalents in fish) and erythrocytes were not recognized by these antibodies. Western blotting of solubilised macrophages revealed that two of the hybridoma lines, designated 21G6 and 21F11, reacted with at least five proteins of 80, 104, 110, 140, and > 170 kDa. Immunocytochemistry was performed on histological sections of trout alimentary canal, gill, liver, spleen, and haemopoietic head kidney using antibodies from several of the hybridoma lines, and all of these showed a similar pattern of reactivity in each tissue. In the alimentary canal, for example, immunoreactive material was found in the eosinophilic granular cells, blood vessel margins, mucus in the lumen, and in the columnar epithelial cells. In the gills, epithelial cells and blood vessels also showed intense immunoreactive products, while in the liver, such reactivity was localised in the sinusoids and adherent macrophages. Both the spleen and head kidney had largely homogenous immunoreactivity.