A method for the radioimmunoassay of plasma estradiol is described which does not require extraction and chromatography of the plasma estradiol. Antibodies to estradiol-6(0-carboxymethyl) oxime coupled to bovine serum albumin are used. Interference from binding proteins normally present in human plasma is removed by adding a gross excess of testosterone to each sample, thereby displacing estradiol from all binding proteins except perhaps albumin which binds only weakly. Separation of bound and free 3H-estradiol is by means of active charcoal. Plasma proteins interfere with the separation step but this interference can be corrected for. Because there is no extraction or chromatography, recovery of plasma estradiol is always 100%. Because the procedure for correction of plasma protein interference makes the standard curve and sample directly comparable, the blank is automatically zero. Data indicating the accuracy of the method are presented. Data concerning the specificity of the antisera are given and the limitations in the use of these antisera are discussed.