A procedure is described for the preparation of an estrone–protein conjugate. The 17-amino derivative of estrone was prepared and converted by phosgenation to estrone-17-isocyanate. The latter was coupled to rabbit serum albumin. Ultraviolet analysis and determination of free amino groups of both albumin and estrone–albumin conjugate showed that the latter contained about 25 estrone residues per albumin molecule. Electrophoretic analysis provided additional evidence that coupling had occurred at the ε-amino groups of the lysine residues of albumin.