A method involving the use of triple-labelled derivatives has been developed for the determination of total oestrone and oestradiol in the plasma of the domestic fowl. The double-labelling technique devised by Svendsen (1960) for the determination of free oestrogens in human plasma was modified to enable the total oestrogen recovery to be determined for each sample. 2. [6,7-(3)H(2)]Oestradiol-17beta is added to the plasma samples (1-10ml.), which are hydrolysed with acid and the phenolic steroids then extracted and partially purified. The extract is esterified with iodobenzene-p[(35)S]-sulphonyl chloride of high specific activity. After addition of standard oestrogen [(131)I]iodobenzene-p-sulphonates the esters are finally purified by paper chromatography. 3. The oestrogens are determined by comparing the (3)H/(35)S and (131)I/(35)S ratios in the purified esters with similar ratios of appropriate standards. 4. With this procedure the recoveries of oestrone and oestradiol after hydrolysis were 70-85% and 72-84% respectively, and after hydrolysis and preliminary purification 38-53% and 39-51% respectively. With this procedure up to 500ng. of oestradiol can be determined. The sensitivity of the technique for oestrone is 3.0ng. and for oestradiol 2.1ng. 5. The ranges of oestradiol and oestrone concentrations found in six plasma samples were 8.3-21.4ng./ml. and 15.2-31.6ng./ml. respectively.
