The development and evaluation of a radioimmunoassay for oxytocin is described. High titre antisera were raised to oxytocin coupled to thyroglobulin and tested for specificity with a number of oxytocin analogues and fragments. Two antisera showing high specificity were used to assay plasma directly without extraction. The maximum sensitivity was 0.5pg per tube and the intra- and inter-assay co-efficients of variation were 7.1 and 11.6% respectively. Cross-reactivity studies indicate that the antisera were directed chiefly to the oxytocin side-chain. The antisera could be purified by affinity chromatography using this tripeptide coupled to agarose beads but this did not improve their avidity or specificity. The assay was tested successfully with a number of body fluids and tissue extracts, although human late-pregnancy plasma could not be added directly to the assay. Direct radioimmunoassay was used to estimate the clearance of oxytocin infused into conscious dogs, and good agreement was found when the same samples were also bioassayed. The antisera also efficiently neutralise the biological activity of oxytocin in vivo and in vitro.