ItL-A antigens prepared from human lymphoid cells by treatment of either intact cells or membranes with papain or nonionic detergents have been shown to contain two polypeptide chains. The antigen prepared by treatment of cells or cell membranes with papain contains polypeptide chains with a mol wt of about 34,000 and 11,000 (1), whereas the detergent-solubilized antigen has polypeptides of tool wt about 43,000 and 11,000 (2). Another membrane-bound protein found on human lymphocytes is ~2-microglobulin, an 11,700 mol wt polypeptide which was first isolated from the urine of patients with renal tubular disease (3) and which has subsequently been sequenced and shown to possess a moderate degree of sequence homology to constant region domains of immunoglobulin polypeptide chains (4, 5).