Thrombomodulin has been confirmed to be able to accelerate the activation of protein C as a thrombin cofactor. This paper reports an assay on protein C in human plasma samples based on the ability of activated protein C to prolong the kaolincephalin activated partial thromboplastin time of normal plasma. Protein C is first separated from its inhibitor by elution of a barium citrate precipitate, and then activated through incubation with thrombin and thrombomodulin. The length of the coagulation time after recalcifieation in the assay serves as a measure of the anticoagulant activity of protein C. When the value in pooled normal plasma was expressed in percentage, the value obtained by this method was proved to be 97±35% in 72 normal control, 178±43% in 30 cases of systemic lupus erythematosus and 116±46% in 12 cases of cerebral stroke. The value of systemic lupus erythematosus, was significantly higher than that of the control(P<0.001), but that in the cases of cerebral stroke showed no remarkable difference. The within-coefficient variation was 6.57, and between-coefficient variation was 15.2. This study has proved protein C assay to be a relatively simple, rapid and less factor-affected method and suitable for routine application. The indices of antithrombin-Ⅲ antigen and activity, Ⅷ:Ag, Fbg/FDP and CIC were also determined in the 30 cases of systemic lupus erythematosus. The role of protein C in thrombosis and hemostasis of the systemic lupus erythematosus patients was discussed in the paper.