We investigated annexin V expression and membrane vesiculation during activation of four leukemic cell lines (U937, HL60, HEL, and CMK11-5) in order to determine whether annexin V had a role in the coagulation abnormalities related to malignancy. After stimulation by tissue plasminogen activator, binding of a monoclonal anti-annexin V antibody to U937 cells and HL60 cells increased in comparison with binding to control cells. Stimulation with thrombin or lipopolysaccharide also induced such an increase, but U46619 did not. Following activation of U937 and HL60 cells with thrombin and lipopolysaccharide, microparticle formation increased. Tissue plasminogen activator caused an increase of microparticles in U937 cells, but not HL60 cells. On the other hand, CMK11-5 and HEL cells did not show any increase of microparticles. These results suggest that some agonists can potently stimulate expression of prothrombinase