We prepared the organometallic complex 17a-(ferrocenylethynyl)estradiol ( [(3,17b-dihydroxyestra-1,3,5(10)-trien-17a-yl)ethynyl]ferrocene; FcEE; 1) by a novel synthetic method. This metallocene possesses sufficient stability in aqueous media to permit the study of its biological properties. Thus, we were able to show that, despite the addition of a bulky substituent at the 17a position of the steroid, the metallocene is still wellrecognized by an antibody specific to estradiol (CR 40%) and by both subtypes (ERa, ERb) of the estrogen receptor (at 08, RBA 28 and 37%, resp.). A DCI-MS study of the stability of the carbocation [FcEE À OH] showed moderate stabilization of the carbocation, in agreement with the pK R value of À 0.72 found for the metallocene by means of Deno×s method. The presence of the ferrocene allows the electrochemical detection of FcEE (1) by HPLC-ED, with a detection limit of ca. 1 nm, suitable for quantitative pharmacological analysis.