At low oestrogen concentrations the assay methods of Brown (1955) and of Bauld (1956) for measuring oestriol, oestrone, and oestradiol-17β in urine are limited by impurities remaining in the final fractions. This interference has been diminished in Brown's method by an additional step of purification taken from Bauld's method, thus enabling the three oestrogens to be measured in most urines from men and pre-and postmenopausal women (Brown, Bulbrook & Greenwood, 1957 a). However, more complete purification is necessary for the measurement of still smaller amounts of oestrogens present, for example, in the urine of children, or of patients who have been subjected to gonadectomy, adrenalectomy or hypophysectomy. A procedure giving highly purified oestrogen fractions in quantitative yield would be useful in the study of oestrogen excretion after the administration of small doses of tritium-labelled precursors. Such a procedure would obviate the necessity of purifying to constant specific activity with its