We have characterised the Procoagulant activity (PCA) of six well-established cell lines by assays of tissue factor (TF), thrombin and FXa generation, flow cytometry using Annexin V, and by binding studies with Factors VIII and IXa. The monocytic (THP-1 & U937) and promyelocytic (NB4) cells expressed high concentrations of TF antigen and activity, whereas TF in the lymphocytic cells (Molt 4, Jurkat & Nalm 6) was very low or absent. However the T-lymphoblastoid cells (Molt 4 & Jurkat) promoted the generation of large amounts of thrombin despite their low TF content, and these cells were also the most active in supporting Factor Xa generation. Molt 4 cells bound Factors VIII and IXa with high capacity and their activity was inhibited by Annexin V. These results indicate that the PCA of T-lymphoblastoid lines is due to expression of negatively charged phospholipids. Flow cytometry studies showed Annexin V binding to the major population of nonapoptotic Molt 4 cells and the PCA of Molt 4 was not increased when apoptosis was induced by staurosporine, indicating that PCA is independent of apoptotic status.