Abstract Tissue factor activity and antigen were measured in promyelocytes freshly isolated from a patient with acute promyelocytic leukemia (APL), FAB M3. Determination of functional activity revealed that physically disrupted cells expressed considerable tissue factor of which less than two percent was available prior to physical disruption of the cells. No tissue factor antigen was detectable on the cell surface by fluorescence flow cytometry. In contrast, endotoxin-stimulated peripheral blood monocytes and monoblastic cells isolated from a patient with monoblastic leukemia had notable populations of tissue factor-positive cells by flow cytometry, and expressed higher proportions of total tissue factor activity without disruption. While some cell types may express both tissue factor antigen and activity when intact, others, which can be extremely rich in tissue factor, may express neither antigen nor activity without a triggering event such as cell damage.