The MRC OX-45 cell surface antigen is a glycoprotein of 45,000 apparent mol. wt of rat leukocytes and endothelium. Antibodies against the antigen inhibit T lymphocyte responses by stimulation of suppression by accessory cells. We now report the immunochemical characterization of this antigen and its cDNA sequence. The predicted protein sequence contains 240 amino acids including a leader sequence of 22 residues and a carboxy-terminal sequence of 23 residues that is replaced in the processed molecule by a glycosyl-phosphatidylinositol anchor attached at serine 195. Two Ig-related domains are predicted to account for all of the processed sequence and the circular dichroism spectrum shows pure beta-structure. The amino-terminal domain is V-like, but without a disulphide bond, while the second domain is C-like (C2-SET) with two disulphide bonds. The sequence matches particularly well with the extracellular parts of LFA-3 and CD2 antigens and the first two domains of carcinoembryonic antigen and non-specific, cross-reacting antigen.