To induce a good immune response to oxytocin (OT) we developed a two-step technique to conjugate OT to thyroglobulin (TG) using glutaraldehyde. We obtained 30 hybridomas recognizing OT-ovalbumin conjugates and 16 stable lines. Three monoclonal antibodies were selected for further characterization. One of them (O13) was found very specific for OT using three different techniques (enzyme-linked immunosorbent assay, radioimmunoassay and immunohistochemistry); it is directed to the C-terminal tripeptide. The other two probably recognize tyrosine containing epitope(s) also shared by vasopressin and other related nonapeptides.