Intact bovine fibroblasts, pericytes, and kidney cells manifested significantly less tissue factor procoagulant activity than their disrupted counterparts. Addition of calcium ionophore A23187 rapidly and reversibly enhanced the cell-surface expression of tissue factor in intact cells up to the level achieved by disruption. Inhibitors of calmodulin blocked the ionophore-dependent enhancement of procoagulant activity. Similar kinetic parameters were obtained for factor X hydrolysis by tissue factor-factor VIIa on unperturbed pericytes and phosphatidylcholine vesicles. Increase in Vmax and decrease in apparent Km for this reaction were seen after either disruption or ionophore stimulation of the pericytes. Addition of phosphatidylserine to the reconstituted phospholipid vesicles also increased the Vmax and decreased the apparent Km for factor X hydrolysis. These data agree with the hypothesis that the expression of tissue factor procoagulant activity on cell surfaces is modulated by calcium-mediated changes in the asymmetric distribution of phosphatidylserine in plasma membrane.