Factor VIIa binding to fibroblasts increases tissue factor expression at the cell surface. Fibroblasts were incubated with a control buffer, FVIIa (10 nM), or FFR-FVIIa (10 nM) for 2 hours at 37°C. Thereafter, the cells were washed with 5 mM EDTA to remove the bound FVIIa/FFR-FVIIa, and cell-surface TF levels were determined by incubating the cells for 2 hours at 4°C with ¹²⁵I-TF mAB (TF9H10) or ¹²⁵I-FVIIa (10 nM) (± polyclonal anti–human TF) and then determining TF mAB binding (B) or TF-specific FVIIa binding (C) to the cells. To measure TF functional activity, fresh FVIIa (10 nM) and factor X (175 nM) were added to the cells and the rate of factor X activation was measured (A). Cell-surface TF antigen and activity were significantly higher statistically (P < .003 to .008) in FVIIa-treated cells compared with control vehicle–treated cells or cells treated with FFR-FVIIa. No significant differences were found between control vehicle– and FFR-FVIIa–treated cells (n = 3 to 6, mean ± SEM).