Protein S is a vitamin K-dependent coagulation factor involved in the regulation of the anticoagulant activity of activated protein C (APC). Heterozygosity for hereditary protein S deficiency is considered as a risk factor for the development of thrombotic disease. To measure the plasma concentration of protein S, two enzyme-linked immunosorbent assays (ELISA) have been developed: the ELISA-p (using polyclonal antibodies both as catching and tagging antibodies) and the ELISA-m (using polyclonal antibodies as catching antibodies and a monoclonal antibody as tagging antibody). The performance of these assays has been compared with that of an immunoradiometric assay (IRMA) for total protein S. Results obtained with both ELISA's in plasma's of healthy volunteers, patients with hereditary protein S deficiency type I and patients using oral anticoagulant treatment correlate very well with those of the IRMA: r = 0.974 (n = 57) for ELISA-p and r = 0.966 (n = 57) for ELISA-m. The intra- and inter-assay variation (n = 6) were calculated to be 5% and 11% for the ELISA-p and 4% and 10% for the ELISA-m. Both ELISA's can be used for the measurement of total protein S, but also for the measurement of free protein S and C4b-BP-PS complexes, after PEG precipitation. The ELISA-m was further used to measure free protein S in healthy volunteers (n = 40), patients with a protein S deficiency type I (n = 20) and patients with unexplained familial thrombosis (n = 76). In the first two groups measured and calculated free protein S levels were very similar. In the third group 7 patients were identified with total protein S in the normal range, but with measured free protein S and the ratio between the measured free protein S and calculated free protein S below the lower limits of the normal range.