We developed a sensitive tissue factor (TF) chromogenic assay on a limited number of endothelial cells (EC), performed in microtiter plates, and which uses normal pooled human plasma instead of purified concentrates as a source of coagulation factors. Primary cultures of human umbilical vein EC (HUVEC), both unstimulated and stimulated by lipopolysaccharide (LPS), were incubated with 50 μl of diluted normal human plasma (NHP) and 50 μl of Factor Xa-specific chromogenic substrate (CBS 31–39, Stago, France). Hirudin was added at 4 U/ml to the plasma/CBS 31–39 mixture to inhibit thrombin generation. Optical densities were read at 405 nm and corresponding amounts of generated factor Xa were expressed in mU Xa/well using a standard curve established with purified human Factor Xa. The following parameters were then defined: the number of EC to plate (10 4 EC/well of a 96-well plate), the plasma-test dilution (1:20), the concentration of CBS 31–39 (0.50 mM) and the incubation time of reagents with EC (2 hours). The procoagulant activity (PCA) measured was only dependent on TF since it was no longer detectable either when FVII-deficient plasma was tested instead of normal human plasma or when PCA assays were performed in the presence of a blocking anti-human TF monoclonal antibody. This method allowed detection of a TF-dependent PCA on as few as 1000 EC per well. In addition, TF expression equal to 50% of maximal values was measured with LPS concentrations as low as 1 ng/ml, supporting the high sensitivity of the assay.