The synthesis of 17␣-ruthenocenyl-17-oestradiol and results of biochemical tests to determine its suitability as a radiopharmaceutical agent, are reported. 17␣-Ruthenocyl-17-oestradiol was obtained, in an overall yield of 29%, by addition of ruthenocenyl-lithium (prepared by treatment of ruthenocene with t-butyl-lithium) to the ketone function of protected oestrone, followed by the deprotection of the 3-OH function. It was characterized by X-ray crystallography: space group P 2 1 (monoclinic), a = 9.150(2) Å, b = 11.806(4) Å, c = 12.193(3) Å,  = 94.56(2)°, V = 1313(2) Å 3 , Z = 2. The relative binding affinity (RBA) of this complex for the oestradiol-specific receptor was compared with that of oestradiol. 17␣-Ruthenocyl-17oestradiol is still recognized by the oestradiol receptor with an RBA of 2%. Unlike its analogue, 17␣-propynyl-Co 2 (CO) 6 -17-oestradiol, it does not act as an affinity marker for the oestradiol receptor. This may be explained by the relative stability of the carbenium ion generated from it, which has a pK R + value of ؉ 0.73. 17␣-Ruthenocyl-17oestradiol is however of potential interest as a radiopharmaceutical agent since ruthenium has radioactive isotopes emitting and ␥radiation useful in nuclear medicine.