Tissue factor (TF) is known to be produced by monocytes from human peripheral blood. However the production of this factor by haematopoietic progenitor cells is not yet known. We thus studied human monocyte progenitor cells isolated from bone marrow of normal and diseased individuals. These cells were non-adherent, monocytic and able to phagocytose particles ranging from 0.3-1 microns. Unactivated partial thromboplastin time clotting assay demonstrated procoagulant activity consistent with TF function, which was blocked by a neutralizing anti-TF monoclonal antibody, G12. The production of TF messenger RNA was demonstrated on dot blot and northern blot analysis utilizing an oligonucleotide probe.