The unexpected discovery that la-like (HLA-DR) antigens^1–3 in humans were present on blast cells from acute myeloblastic leukaemia^4,5 led to the finding that normal granulocytic progenitors, in contrast to their mature descendents, also expressed HLA-DR antigens. Thus, anti-Ia sera stain a proportion of myeloblasts in normal bone marrow^6,7, inhibit myeloid progenitor (CFU-GM) colony formation in the presence of complement⁸ and can be used to label and separate CFU-GM on a fluorescence-activated cell sorter (FACS)⁹. Winchester et al. subsequently reported that erythroid progenitors (BFU-E and CFU-E) were also inhibited or killed by anti-Ia (p28,37) and complement¹⁰. These observations raised the possibility that HLA-DR (or presumptive I-region equivalent) products might have a regulatory role in early haematopoiesis. We have now analysed HLA-DR and HLA-ABC antigen expression on normal erythroid progenitors using monoclonal antibodies to non-polymorphic determinants and fluorescence-activated cell sorting. In parallel experiments, we tested a monoclonal antibody to glycophorin, a well defined erythroid-speciflc cell-surface membrane glycoprotein¹¹. We report that HLA-DR, HLA-ABC and glycophorin are all expressed at various stages during erythroid differentiation.