Fas antigen (CD95) is a cell surface receptor belonging to the tumour necrosis factor/nerve growth factor superfamily and is able to induce apoptosis when triggered by its' natural ligand or an anti-Fas antibody. Fas expression is low on CD34+ bone marrow (BM) progenitor cells, but is increased by various cytokines in vitro. We investigated Fas expression on CD34+ cells from 39 peripheral blood progenitor cell (PBPC) harvests and from 5 normal BM harvests by dual colour flow cytometry to determine if Fas expression was altered during mobilisation. By including calibrated microbeads during flow cytometry, we quantified the number of Fas antigen molecules per cell. A low percentage of PBPC (22%) and normal BM (23%) CD34+ cells expressed Fas antigen. Fas expression varied on CD34+ cells from different diseases and the highest expression was found in ALL (52%). There was a significant three fold increase in the number of Fas molecules/cell expressed on CD34+ cells (PBPC 6,230 molecules/cell, BM 2,236; p = 0.0003). This level of expression was considerably less than that for CD3/CD19 lymphocytes (33,095 molecules/cell) and CD14 monocytes (47,467 molecules/cell) in the PBPC harvest. In conclusion, mobilisation including the use of growth of factors, has minimal effect on CD34 progenitor cell Fas expression.