P rotein C is a natural vitamin K-dependent plasma anticoagulant, deficiencies of which have been found in patients with recurrent thrombosis and warfarin-induced skin necrosis. To appreciate more fully the role of protein C in disease states and during oral anticoagulation, a new functional assay for protein C involving adsorption of plasma protein C on a Ca+2-dependent monoclonal antibody, elution, quantitative activation, and assessment of plasma anticoagulant activity, has been developed. When oral anticoagulation is initiated, the anticoagulant activity of protein C decreases to a greater extent than either the amidolytic or immunologic levels. During stabilized warfarin treatment, there is no correlation between either amidolytic or antigenic levels and the functional protein C activity, suggesting that measurement of protein C anticoagulant activity may be necessary to reflect adequately the anticoagulant protection afforded by this protein. In contrast, there was a strong correlation between anticoagulant and amidolytic and immunologic levels in liver failure and disseminated intravascular coagulation. Two patients with thromboembolic disease have been identified who exhibit a marked decrease in anticoagulant activity, but who have normal immunologic and amidolytic levels. Thus, this assay permits assessment of protein C in individuals who have received anticoagulant treatment and identification of a new class of protein C-deficient individuals.