The cell-surface expression and functional status of the CD95/Fas antigen on primitive hematopoietic progenitors isolated from human cord blood (CB) were studied. The CD34+ cells freshly isolated from CB displayed low CD95 expression. The combinations of cytokines such as SCF+FL could up-regulate the expression of CD95 in vitro culture and tumor necrosis factor-e (TNF-α) and interon-γ (IFN-γ) further increased the CD95 expression induced by positive cytokines. The functional status of CD95-mediated apoptosis were analyzed by incubation of CD34+CB cells in the presence of anti-CD95 monoclonal antibodies (McAbs). The effects of anti-CD95 McAbs were measured by viable cell counting, flow cytometry, LTIC and CFU-C assays. A decrease of viable cells, CFUC and LTIC numbers were observed in the presence of anti-CD95 McAbs and TNF-α or IFN-γ.However, growth factor deprivation or the early-acting cytokine such as SCF and FL cross-linking to CD95 caused low apoptosis of CD34+ cells. The correlation of increased intracytoplasmic levels of bcl2 and the presence of CD95 on fresh CB CD34+ cells suggested that bcl-2 might be involved in protecting against CD95-mediated apoptosis of CB CD34+ cells.