The authors describe the technic and standardization of an assay to measure plasma protein C activity feasible for clinical laboratory use. It is modified from an assay of Francis and Patch (Thromb Res 1983; 32:605-613) to enhance protein C recovery in the barium citrate eluate, to eliminate the steps of addition and neutralization of heparin, and to use only commercially available reagents. The normal range for plasma protein C in the assay is 72-130% (+/- 2 SD) (0.7-1.30). Hepatocellular disease lengthening the prothrombin time by 3-4 seconds was associated with plasma protein C activity of 25% (0.25) to 35% (0.35). Although the assay is thought to accurately measure protein C activity in patients taking warfarin, one cannot evaluate such patients for hereditary functional protein C deficiency because treatment with warfarin will itself reduce the ratio of protein C activity to antigen. The assay can be used in a patient receiving heparin if the heparin is removed as described.