Protein C is the zymogen of a vitamin K-dependent serine protease involved in blood coagulation. In the absence of protein C the inactivation of activated factors V and VIIIC is impaired, and the fibrinolytic capacity of the circulating blood is reduced. These conditions promote excessive fibrin formation and thus constitute a risk factor for thrombosis. Using an immunologic assay for protein C, we identified 18 patients (11 male and 7 female) in three unrelated Dutch families as fulfilling the criteria for an isolated protein C deficiency. In 12 patients who were not receiving oral anticoagulant treatment the mean protein C antigen concentration was 0.48 +/- 0.09 U per milliliter (+/- S.D.), and in 6 patients who were receiving adjusted doses of oral anticoagulants and had stable anticoagulation, the mean value was 0.17 +/- 0.05 U per milliliter. (The value in healthy subjects is 0.98 +/- 0.19 U per milliliter.) Fourteen of the 18 patients had a history of venous thromboembolism, with superficial thrombophlebitis as the hallmark of this condition (in 13 patients). These data are consistent with an autosomal dominant trait with variable expressivity.