Measurement of Protein S in human plasma is clinically important because of deficiency of this protein, which functions as a cofactor of the naturally occurring anticoagulant activated Protein C, is a risk factor for venous thromboembolism. We describe a two-site, enzyme-linked immunosorbent assay (ELISA) for measuring Protein S in which a monoclonal IgG directed against the calcium-independent conformation of Protein S is the capture antibody. The range of detection for the assay was 10 to 160 ng of Protein S per milliliter. The coefficients of variation were 4.6%-7.3% within-assay and 7.7%-10.1% between-assay. We compared this assay with an ELISA involving a polyclonal anti-Protein S rabbit IgG as capture antibody (I) and with Laurell's electroimmunoassay (II) to measure Protein S in plasma from 32 normal subjects and 121 patients or individuals expected to have low concentrations of total Protein S (full-term newborns, pregnant women after the 18th week of gestation, patients with disseminated intravascular coagulation or liver cirrhosis, patients receiving therapy with warfarin, and patients with congenital Protein S deficiency). In general, the results obtained with the monoclonal antibody-based ELISA correlated well with those from I (r = 0.94), less well with those from II (r = 0.86).