We have used a new methodology to generate a monospecific antiserum to the corticotropin (ACTH) receptor on mouse Y-1 adrenal cells. Using immunoaffinity chromatography the ACTH receptor was purified, and the molecular structure and 125I-ACTH binding characteristics were determined. A molecular weight (Mr) of 225 000 was determined for the complete ACTH receptor as analyzed by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. The receptor was composed of 4 subunits with Mr 83 000, 64 000, 52 000 and 22 000. The 83 and 52 kDa subunits were disulfide linked and non-covalently associated with the 64 and 22 kDa subunits. The ability to specifically bind 125I-ACTH was localized to the 83 kDa subunit. The purified receptor possessed binding affinities of 3.4 × 1010 M-1 and 1.0 × 109 M-1 as determined by Scatchard analysis. © 1986.