Synthetic oxytocin (OT) conjugated to bovine thyroglobulin by the carbodiimide reaction was injected into rabbits to raise a high titre, specific OT antiserum which was then coupled to microcrystalline cellulose activated by cyanogen bromide. High affinity of the coupled antiserum was defined by Scatchard analysis, Keq = 7.1 X 10(11)1/mol. Cross-reactivity studies revealed little binding of antiserum to analogues of OT. Iodination was performed by the Chloramine T method, giving specific activity of 125I-OT, range 1.1 - 1.7 X 10(3) Ci/mmol. After incubation for 40 hours under disequilibrium conditions, specific and non-specific bindings were 10.6 +/- 2.7% and 0.2 +/- 0.1% (n=15), respectively. Displacement of 50% 125I-OT occurred with 2.9 pg OT/tube. Coefficients of variation of standard OT concentrations (0.03 - 16 pg/tube) were less than 5%. Limit of detection was 2 pg OT/ml plasma. Recovery of synthetic OT added to non-pregnant plasma was 81.8% (n=34) at 20 pg/ml and 97.4% (n=32) at 100 pg/ml. Two patients, 17 and 18 weeks post-partum, had increases in plasma OT from less than 2 pg/ml to 18.3 and 16.0 pg/ml after 6 and 4 minutes breast feeding infants, respectively. We conclude that this solid phase OT radioimmunoassay is quick, relatively sensitive and reliable, and does not require prior extraction of plasma samples.