Protein S, a vitamin K-dependent coagulation factor, is involved in the regulation of the anticoagulant activity of activated protein C. Using an immunoradiometric assay for total protein S in plasma we identified 14 patients (7 male and 7 female) in three unrelated Dutch families as fulfilling the criteria for an isolated protein S deficiency. In 9 patients who were not receiving oral anticoagulant treatment the mean total protein S antigen concentration was 0.50 +/- 0.08 U/ml (+/- S.D.) and the calculated free protein S concentration was 0.15 +/- 0.01 U/ml (+/- S.D.). In the five patients who were on oral anticoagulant treatment the mean total protein S antigen was 0.23 +/- 0.05 U/ml (+/- S.D.). Seven of the 14 patients had a history of venous thromboembolism occurring at a mean age of 25 years and often without an apparent cause. Protein S deficiency is inherited as an autosomal dominant trait.