With the aid of a complex method of purification, including affinity chromatography and HPLC, from a fraction of the water-soluble protons of cotton seeds we have isolated a protein which binds the labeled auxin [3HJIAA with high affinity (Kd = 6 nM) and in the presence of IAA stimulates the synthesis of RNAin vitro in a preparation of cottonplant chromatin. In PAAG electrophoresis with sodium dodecyl sulfate, this protein gave a single band corresponding to a molecular mass of 43 kDa. It is assumed that this protein is a receptor for the auxin phytohormone that is responsible for regulating gene activity.