A sensitive and specific antiserum for ?-endorphin(?-EP)was raised in male New Zealand white rabbits by primary and 5 booster injections with synthetic ?n-EP which was coupled to thyroglobulin with carbodimide.The antiserum has a high titer(1: 16000)and high avidity(detection limit 5 fmol/tube, IC50260 fmol/tube, Kaff about 6.35 x 109 L/mol).A reliable radioimmunoassay was developed.The results show that the method is suitable for measuring immunoreactive ?-EP contents in pituitary, brain areas and plasma of rats.