An enzyme-linked immunosorbant assay (ELISA) for measuring oestradiol directly in plasma without extraction utilizing antibodies raised against oestradiol-3-(O-carboxymethyl) ether-bovine serum albumin conjugate, and oestradiol-6-(O-carboxymethyl) oxime linked to penicillinase (EC 3.5.2.6) as a marker was developed. Polyvinyl 96-well microtitre plates were used for immobilization of anti-oestradiol IgG. Standards of oestradiol (92 to 9,190 pmol/l were prepared in oestradiol-free plasma and 8-anilino-1-naphthalene sulphonic acid (8-ANS, 5 mg/ml of 10 mmol/l PBS) was added to the microtitre plate wells to displace oestradiol from plasma binding proteins. The assay had a lower limit of detection of 92 pmol/l plasma and could be performed within 4 h. Comparison of oestradiol values of 51 plasma specimens obtained by ELISA with those of radioimmunoassay (RIA), in which oestradiol was extracted with diethyl ether, showed good correlation (y = 0.786x + 0.03; r = 0.900).