THE human major histocompatibility complex encompasses at least four well-defined loci. Those called HLA–A, –B, and –C control the expression of the classical transplantation antigens (ref. 1). The fourth locus, HLA–D, controls determinants which regulate the ability to stimulate in mixed leukocyte cultures (MLR) and seems at least partly to correspond to the I-region of the murine H–2 complex. A great deal of work has been done on the immunogenetics of the HLA–D locus (reviewed in ref. 2) hut despite the fact that alloantibodies against HLA–D determinants are available3,4 the molecular nature of the antigens is largely unknown. Such studies have been hampered by the fact that HLA–D determinants only seem to he expressed on a small proportion of the peripheral blood lymphocytes (the B cells)3 but Fu et al.5 have reported data which suggest that B-cell-specific alloantigen is expressed on leukaemic cells. We have therefore isolated a cell membrane glycoprotein complex, controlled by the HLA–D locus, from leukaemic cells of a patient with chronic lymphatic leukaemia. The characteristics of the protein complex strongly suggest that it represents the human equivalent of the murine Ia antigens.