A sensitive, specific and reproducible radioimmunoassay (RIA) for measurement of 3,3'-L-diiodothyoride (T2) in concentrated ethanol extracts of serum and amniotic fluid is described. The T2 binding antiserum was prepared aby immunization of rabbits with T2-human serum albumin conjugate. Of various thyroid hormone derivatives tested, only 3-L-monoiodothyonine cross-reacted significantly with T2 binding sites on the antiserum...