An E.coli strain which could express domain Ⅲ of the extracellular region of the human EGF-R(EGF-RⅢ) in its membrane had been obtained. The results of EGF receptor radiobinding assay showed that ¹²⁵I-EGF could bind specifically to the intact bacterial body, the amount of specific binding varied with the time and temperature of the reaction. The data of Scatchard analysis indicated that the number of the human EGF-RⅢ expressed in the bacteria was about 738 sites/cell, and its dissociation constant was about 3.0×10^-11 mol/L. The sites of the bacteria to which EGF bound were discovered mainly on its membrane by immunoelectronmicroscopy.