Summary. Background: Concentrations of non-cell-bound (NCB; soluble) tissue factor (TF) are elevated in blood collecting in the pericardial cavity of patients during cardiopulmonary bypass (CPB). Previously, we reported microparticles supporting thrombin generation in such blood samples. In this study we investigated the extent of microparticle association of the NCB form of TF in pericardial and systemic blood, and whether this microparticle-associated form is active in thrombin generation compared with non-microparticle-bound, (fluid-phase) TF. Methods: Systemic and pericardial blood samples were collected before and during CPB from six patients undergoing cardiac surgery. Microparticles were isolated by differential centrifugation and their thrombin-generating capacity measured in a chromogenic assay. Microparticle-associated and fluid-phase forms of NCB TF were measured by ELISA. Microparticle-associated TF was visualized by flow cytometry. Results: In pericardial samples, 45–77% of NCB TF was microparticle-associated, and triggered factor VII (FVII)-mediated thrombin generation in vitro. Microparticles from systemic samples triggered thrombin generation independently of FVII, except at the end of bypass (P = 0.003). The fluid-phase form of TF did not initiate thrombin generation. Both forms of NCB TF were, at least in part, antigenically cryptic. Conclusions: We demonstrate the occurrence of two forms of NCB TF. One form, which is microparticle-associated, supports thrombin generation via FVII. The other form, which is fluid-phase, does not stimulate thrombin formation. We hypothesize that the microparticle-associated form of NCB TF may be actively involved in postoperative thromboembolic processes when pericardial blood is returned into the patients.