NORMAL and malignant B cells, some monocytes and blast cells from both the common (non-T, non-B) form of acute lymphoblastic leukaemia (ALL) and acute myeloblastic leukaemia (AML) share Ia-like antigenic determinants1–8. ALL cells, but not AML cells, can be further characterised by the expression of an ‘ALL-associated’ membrane antigen9,10. Although the ALL antigen and the Ia-like antigens on leukaemic blasts have at one time been regarded as candidate leukaemia-specific antigens, it has been suggested that both may be normal differentiation antigens of haematopoietic precursors, their expression in leukaemia reflecting the cellular origins of the disease2,7–11. In immunofluorescent studies combined with phase contrast examination, Ia-like antigens can be demonstrated on normal and leukaemic myeloblasts and some promyelocytes, but not on most promyelocytes and maturing granulocytic or erythroid cells7,8. The presence of Ia-like antigens on myeloid-committed stem cells is supported indirectly by results of functional assays in vitro which show the abolition of myeloid colony-forming cells (CFUc) by incubation with complement and heteroantisera to B cells12,13. Two disadvantages of the approach used in the latter experiments are, first, that the cells of interest are eliminated and no longer available for analysis, and second, that their functional inhibition does not necessarily establish that they carry the antigens in question, as the development of colonies involves stimulatory factors14,15 and auxiliary cells, whose activity, rather than CFUc per se, might be compromised by the antibodies used. We now report direct evidence for the presence of the antigens on bone marrow cells by separating them in a fluorescence-activated cell sorter (FACS), according to their binding of anti-Ia-like (anti-p28,33) or anti-ALL antibodies. Functional analysis of the resultant populations reveals that a proportion of human colony and cluster forming cells express the Ia-like hetero-antigen but not the ALL-associated antigen.