Monoclonal immunoadsorbent chromatography has been used to isolate milligram quantities of detergent-solubilized H-2Kk antigen. Using the procedure described in this paper 1012 cells may be processed yielding 10 mg of homogenous H-2Kk which represents 70% of the allotypic serological activity present in the original homogenate. NH2-Terminal sequence data of the first 30 residues of the H-2Kk heavy chain are presented. The cell line selected as the source of antigen and the criteria of purity of the antigen have been found to be critical as proteins of molecular weight 42,000 and 12,000 were copurified with H-2Kk from the BW5147 cell line. The additional components were observed in gradient gel electrophoresis or twodimensional electrophoresis, but not in conventional Laemmli gel electrophoresis. © 1982.