A radioimmunoassay for ovine calcitonin (CT) using a porcine system and talcum powder separation has been developed. A guinea pig antiserum to partially purified porcine CT was highly specific for CT and was capable of detecting 5 pg of porcine CT. The antiserum cross-reacted significantly with sheep, bovine and horse CT but not with rat, rabbit, human and salmon CT. Plasma CT levels in the jugular vein of pregnant ewes were higher in the last 40 days of gestation than in control ewes. A calcium load given orally (10 mg/kg body weight) in newborn lambs did not change the plasma calcium level but increased the plasma CT concentration.