This report gives details of the authors' procedure for the measurement of 17β-oestradiol in plasma by competitive protein binding analysis, together with the results of studies on various parameters of the assay system. The preparation of the receptor cytosol, the assay system, extraction of plasma and reliability criteria are all discussed. The procedure is compared with those reported by other workers. Finally the results obtained by this technique are compared with those obtained by other methods and current applications are presented.