The Protein C pathway plays a crucial role in the regulation of thrombosis and inflammation. One of the tools that researchers presently use to elucidate mechanisms of action of activated protein C (APC) is the use of transgenic or gene deletion murine models. To correlate observations in these murine models with the APC levels, there is a need for a sensitive and specific assay for circulating murine APC in plasma. We developed an immunological assay to measure the physiological and pharmacologic levels of circulating murine APC. The sandwich ELISA uses an anti-murine anti-protein C antibody capture antibody and human protein C inhibitor (PCI) as a detection reagent, taking advantage of the facts that the mouse lacks plasma PCI and that human PCI forms a 1/1 stable complex with mouse APC. The amount of complex APC:PCI is detected with an anti-human PCI monoclonal antibody. The assay shows improved sensitivity versus enzyme immunocapture assays commonly used to detect human APC and considerably reduces the processing time.