An antibody to porcine calcitonin (CT) was selected which cross-reacted completely to bovine CT in a radioimmunoassay using a synthetic porcine CT preparation (CT-s) labelled with 125I (Lequin, Hackeng, Schopman & Care, 1970). With this radioimmunoassay it has been possible to measure CT concentrations in samples of bovine peripheral and thyroid venous plasma. All concentrations of CT reported here are expressed in terms of immunologically active CT-s/ml. The standard error of the CT measurements was ± 50 pg CT-s/ml over the range of concentration 250–2000 pg/ml.
One lobe of the thyroid gland was isolated in situ in each of three calves (33–38 kg body wt) and in one adult Jersey cow (300 kg body wt). The glands were perfused with blood which contained various concentrations of calcium. Glucagon and the kallikrein inhibitor (KI) aprotinin (Zymofren) were also added in an experiment with the cow to give perfusing concentrations of