The expression of progenitor cell-associated antigen CD34, defined with monoclonal antibody BI-3C5, was investigated in cells from 109 patients with leukaemia. No reactivity was found in chronic leukaemias, whereas 31% of acute myelogenous leukaemia (AML) and most non-T, non-B acute lymphoblastic leukaemia (ALL) expressed CD34. Examples of BI-3C5+ AML included M1 and M2 FAB types only; all but one were myeloperoxidase positive. In combination with pan-myeloid markers, BI-3C5 is useful for identification of immature myeloid cells.