Using labelled ligand-binding methods, previous studies have identified specific angiotensin II receptors (Ang II-Rs) in eel liver, kidney and intestine membranes. Isoelectric focusing on polyacrylamide gels also showed that there are two Ang II-R isoforms in eel liver, focusing at isoelectric points (pI) 6.5 and 6.7. These may have different functions. In contrast, eel enterocyte plasma membrane and renal brush border membranes contain only the pI 6.5 form. To characterize the eel receptors more fully, a newly developed monoclonal antibody (6313/G2) which selectively recognizes the AT1 subtype of mammalian Ang II-R was used. In ligand-binding experiments, the preincubation of eel liver membranes with 6313/G2 antibody eliminated the specific [3,5-3H]Tyr4-Ile5-Ang II binding. Moreover, Ang II-receptor complexes from solubilized liver membranes, which were immunoprecipitated by 6313/G2-coated beads, had a pI of 6.5. In immunoblotting experiments, the antibody recognized the isoform focusing at pI 6.5 in eel intestine and liver preparations, but not the liver pI 6.7 isoform. Immunoblotting of SDS gels showed that the antibody bound to a single protein of molecular mass of 75 kDa in eel liver, gill and kidney and to a doublet of molecular mass of about 74 and 75 kDa in intestinal membrane preparations. Immunocytochemistry of paraffin-embedded and cryostat sections of eel liver, kidney, intestine and gill showed that antibody 6313/G2 bound to uniformly distributed intracellular sites and cell surface membranes in proximal tubular cells, absorptive intestinal cells, hepatocytes and chloride cells. It also stained endothelium and both the longitudinal and circular layers of smooth muscle cells in the intestine. The data suggest that the previously described Ang II-R from eel liver, kidney and intestine may be similar to the mammalian AT1 subtype.