A rabbit antibody against human protein C was used for the quantitative estimation of protein C in plasma. In healthy individuals protein C antigen ranged from 0.65-1.45 U/ml. Plasma protein C antigen was found to be independent of either age or sex. Under influence of oral anticoagulant treatment the protein C antigen concentration decreased to 0.47 U/ml (at low intensity treatment) or 0.33 U/ml (at high intensity treatment). Using normal ranges of protein C and protein C/factor II and protein C/factor X ratios criteria were developed for the assessment of protein C deficiency. In a Dutch family with a history of thrombotic disease two members were found to have an isolated protein C deficiency, while a third one is suspected of protein C deficiency. In one case it was possible to confirm the diagnosis of suspected protein C deficiency during temporary withdrawal of the anticoagulant therapy.