Four monoclonal antibodies (MAbs : Mm, MGm, MGcp1 and MGcp2) were produced against peritoneal exudate cells of rainbow trout, Oncorhynchus mykiss. The MAbs were characterized by immunofluorescence assay, flow cytometry, immunoelectron microscopy and Western blotting, and utilized to isolate macrophages and neutrophils by magnetic cell sorting. The immunofluorescence assay and flow cytometry showed that each of the MAbs was specific for both macrophage and neutrophil populations and these cells were observed in leucocytes from various origins, i.e. the head kidney, peripheral blood and peritoneal exudate. Mm and MGm reacted with membrane antigens, while MGcp1 and MGcp2 reacted with intracellular antigens. The MAb-positive cells were identified as both macrophages and neutrophils by electron microscopy using immunogold labelling. A Western blot analysis revealed that the MAbs detected different determinants (Mm; 205, 151 and 107 kDa : MGm; 205, 143, 107, 76, 55 and 52 kDa : MGcp1; no reaction : MGcp2; 64 kDa). Macrophages and neutrophils were efficiently isolated by a magnetic cell separation system with Mm and MGm. These antibodies will be useful for identifying and isolating rainbow trout macrophages and neutrophils, which facilitate studies of trout leucocytes.