Somatic-cell fusion of normal antibody-producing spleen cells with cells from a plasmacytoma culture results in a culture of hybrid cells from which a monoclonal line may be selected. These lines are immortal and may be amplified as tumors in syngeneic animals to produce large quantities of antibodies characterized by molecular homogeneity. We report the application of this technique to the production of antibodies binding canine renin. Balb/c mice were immunized with pure canine renal renin and their spleen cells fused with the NS-1 myeloma line. In two separate fusions, nine clones of cells were isolated that bound canine renal renin but did not cross-react with a number of protein antigens tested. One of these antibodies cross-reacted with renins of several different species, including human renin. Binding inhibition studies carried out with one of these monoclonal antibodies demonstrated a dissociation constant for renin of 10(-7) M. These monoclonal antibodies have great potential in answering significant questions concerning the structure, biosynthesis, tissue localization, and physiologic actions of renin.