Elimination of peripheral T cells is crucially regulated via apoptosis through CD95 (APO-1/Fas) receptor ligand interaction. Because homeostasis in hematopoietic cells may also involve the CD95 system, we analyzed CD95 expression and sensitivity to CD95-induced apoptosis in human bone marrow cells. During hematopoiesis CD95 is differentially expressed on distinct cell types and at different maturational stages with an increase in receptor density from early CD34+ stem cells to maturing progenitor cells. Incubation of bone marrow cells with anti-APO-1 (anti-CD95) induces apoptosis in maturing erythroblasts and neutrophil progenitors (10-19%) and to a lesser extent in stem cells and myeloblast/proerythroblasts (4-9%). On in vitro culture, CD95 expression is particularly upregulated on activated CD71+ myeloid progenitors. Hematopoietic cytokines (stem cell factor, interleukin-3, granulocyte macrophage colony-stimulating factor [GM-CSF], and granulocyte-colony-stimulating factor [G-CSF] contribute to upregulation of CD95 on bone marrow cells. CD95-induced apoptosis in activated progenitors was markedly enhanced by activating cytokines. Thus cytokines known to mediate proliferation, survival, and maturation, in hematopoiesis do not prevent, but rather facilitate negative growth regulation via the CD95 pathway in activated cells. Deregulation of the CD95 system may provide a molecular basis for the development of bone marrow failure or immune-mediated cytopenia. Defects in the CD95 pathway may contribute to the development of hematopoietic malignancy by abrogating CD95-mediated growth control of activated cells.