Coagulation is initiated on tissue-factor-bearing cells when factor VIIa complexes with membrane-bound tissue factor and activates factors X and IX. Cellular tissue factor activity does not correlate with tissue factor antigen; treatment with calcium ionophore rapidly increases tissue factor activity without increasing tissue factor antigen. Our study examined the effect of calcium ionophore A23187 on tissue factor activity of freshly isolated, lipopolysaccharide-stimulated monocytes and non-transformed human dermal fibroblasts. A23187 increased tissue factor activity on monocytes and fibroblasts in a dose-dependent fashion between 0.1 and 50 micromol/l ionophore. This increase in activity was proportional to an increase in intracellular calcium in monocytes. The increase in tissue factor activity was partially attributable to an increase in phosphatidylserine expression, as measured by increased prothrombinase activity (1.1- to 4-fold) on ionophore-treated cells. The phosphatidylserine-binding protein annexin V decreased tissue factor activity on both ionophore-treated and untreated cells, reflecting the role of phosphatidylserine in tissue factor activity. However, even in the presence of saturating concentrations of annexin V, the tissue factor activity of ionophore-treated cells was 1.3- to 11.3-fold higher than that of untreated cells, indicating that the increase in tissue factor activity did not result solely from increased expression of phosphatidylserine. A23187 increased tissue-factor-dependent activation of factors IX and X 1.4- to 7-fold on both cell types, indicating that ionophore treatment did not alter factor VIIa/tissue factor substrate specificity. We conclude that the mechanism by which calcium ionophore increases tissue factor activity is not unique to monocytoid or transformed cells. Furthermore, the ionophore-induced increase in activity is not solely the result of increased exposure to phosphatidylserine. Finally, tissue factor de-encryption by A23187 does not alter factor VIIa/tissue factor substrate specificity.