Leukaemic cells show a low clonogenic activity and a heterogeneous proliferative response to growth factors. We investigated whether this could be due to an altered expression of growth factor receptors on the leukaemic precursors. Receptors for G-CSF, stem cell factor (SCF), IL-3, IL-6 and IL-7 were detected on CD34+ cells in AML and B-lineage ALL with monoclonal antibodies and flow cytometry. The expression was compared with that on myeloid and B-lymphoid CD34+ cells in normal bone marrow. Leukaemic CD34+ cells expressed the same receptors as their normal counterparts. AML and B-lineage ALL could be distinguished by the growth factor receptor profile of their CD34+ cells. SCFR, G-CSFR and IL-6Ralpha were found in AML, IL-7R in B-lineage ALL and IL-3Ralpha in both. IL-3Ralpha was upregulated in AML and B-lineage ALL CD34+ cells, while samples with low or high expression were present for the other receptors. This variable expression could correlate with the heterogeneous response of leukaemic cells to growth factors. Functional studies on isolated CD34+ cells are needed to investigate this further.