1. A vasopressin binding protein purified from rat liver membranes was used to immunize Balb/c mice and, subsequently, for the screening of hybrids raised in two different cell fusions. 2. Three hybrids were obtained which secreted monoclonal antibodies (MoAb) that bound to the purified solubilized receptor as detected by an enzyme-linked immunosorbent assay technique. All three MoAb immunoprecipitated the purified receptor. 3. In addition, the MoAb bound in a concentration-dependent manner to crude liver, kidney and anterior pituitary membranes, tissues known to contain arginine vasopressin (AVP) receptors but not to cardiac ventricle membranes which lack AVP receptors. 4. However, the binding of [125I]-[d(CH2)5,Sar7]AVP (a specific radiolabelled V1 antagonist) to the membrane-bound receptor was not inhibited by these antibodies. 5. These results suggest that MoAb recognize epitopes which are common to rat liver, kidney and anterior pituitary membranes but are not at the ligand binding site.