Particulate fractions of guinea-pig thyroid homogenate contained a single class of receptors which bound 125I-labelled bovine thyrotrophin (TSH) by a saturable, reversible process with an affinity constant of 2 \ m=x\ 109 1/mol. The binding process was specific for TSH, and corresponded with the activation of adenylate cyclase. Cleavage of hormone\p=m-\receptor bonds by treatment with lyotropic agents resulted in the release of unchanged labelled TSH. The radioligand receptor assay system was sensitive to 0\m=.\015mu. TSH. Bovine or mouse thyroid showed reduced binding affinity with correspondingly reduced sensitivity.

1974): Correlation between long~acting thyroid stimulator protector level and thyroid 131 I uptake in thyrotoxicosis. Brit. me d. 1974): Stimulation of the human thyroid by infusions of plasma containing LATS protector, J. c/in. Endocr. 39,826. 7. McKENZIE, J. M. (1968): Delayed thyroid response to serum from thyrotoxic patients. Endocrinology 62, 865. 8. ADAMS, D. D. ( 1958): The presence of an abnormal thyroid-stimulating hormone in the serum of some thyrotoxic patients, J. c/in. Endocr. 18, 699. 9. KNIGHT, A. and ADAMS, D. D. (1973): Infusion of LATS protector and LATS into monkeys, Proc. Univ. Otago med. Sch. 51, I I. 10. WALL, J. R., GOOD, B. F. and HETZEL, B. S. (1969): Long-acting thyroid stimulator in euthyroid relatives of thyrotoxic patients, Lancet 2, I 024. I I. CHOPRA, I. : A radio-ligand receptor assay for the long-acting thyroid stimulator, Biochem. J. 14!, 105. 15. KRISS, J.P., PLESHAKOV, V. and CHIEN, J. R. (1964): Isolation and identification of the long-acting thyroid stimulator and its relation to hyperthyroidism and circumscribed pretibial myxoedema, J. c/in. Endocr. 24, 1005. 16. 0DDJE, T. H., MESCHAN, I. and WORTHAM. J. (1955): Thyroid function assay with radioiodine. II. Routine calculation· of thyroidal and renal rate factors,

SUMMARY 125I-Labelled thyrotrophin (TSH) showed saturable binding to hyperplastic guinea-pig thyroid slices in vitro, but not to kidney, adrenal, liver, testis or salivary gland slices. Two populations of binding sites were demonstrated: high order sites showed an affinity constant of 3·8 × 1081/mol and a capacity of 8 × 103 molecules of TSH/cell; low order sites, affinity constant of 2·9 × 1071/mol and capacity of 8 × 104 molecules/cell. The highorder sites saturate at about 1 mu./ml, approximately the level producing maximal in-vivo and in-vitro responses. Binding of 125I-labelled TSH was reversible, the released hormone appearing substantially unchanged as assessed by chromatographic behaviour and antiserum binding. Release of unlabelled TSH from pre-incubated thyroid slices was demonstrated by bioassay. Bovine thyroid slices were found to have a lower capacity for binding than hyperplastic guinea-pig slices.

1. When oxytocin and vasopressin are incubated at 37° C. for two hours with suitable volumes of plasma from healthy pregnant women, or with saline extracts of normal or pre‐eclamptic placentæ, they are destroyed. 2. The loss of both oxytocic and antidiuretic activity is thought to be due to an enzyme which is heat labile with an optimum pH between 7 and 8. 3. Placentæ from healthy pregnant women contain more enzymatic activity than placentae from women with pre‐eclamptic toxæmia.