A radioimmunoassay (RIA) for thyroliberin (TRH) is described which differs on several points from the previously published ones. A TRH antiserum was raised in rabbits against TRH coupled to a seed-globulin from the sun-flower; [(3)H]TRH was used as radioligand and precipitation of the antibody-antigen complex was achieved with polyethylene glycol (PEG). The RIA sensitivity is around 4 pg. It is compared to a radioreceptor assay (RRA) which has been previously settled  using a pituitary cell line in culture (GH3 clone) (sensitivity: 200 pg). No cross-reactivity is seen in either assay with various hormones or TRH analogues, except with some methylated derivatives. Both techniques led to comparable values when biological extracts were assayed, provided they were partially purified.