1. Balb/c mice were immunized against a vasopressin binding protein purified from rat liver. The hybrids produced from two cell fusions were screened against this receptor. Three hybrids were selected, cloned and expanded in serum-free media. The monoclonal antibodies (MoAb) secreted by these three hybrids were of the subclass IgM and were able to immunoprecipitate [125I]-labelled purified receptor. 2. All three MoAb bound to the purified solubilized receptor, crude liver and kidney membranes in a concentration-dependent manner. However, the binding of MoAb to the membranes did not inhibit the binding of [125I]-[d(CH2)5,Sar7]AVP, a selective V1 receptor radioligand, to the liver membrane-bound receptor. 3. These results suggest that the three MoAb recognize epitopes on the V1 receptor which are not denatured by solubilization, but are common to both rat liver and kidney membranes.