The fibrin lysis activity in the presence of enough quantity of C1 inactivator (C1.INA) is called C1.INA resistant fibrinolytic activity (C1.IRFA). C1.INA inhibits the activation of intrinsic fibrinolysis, therefore C1.IRFA is considered to reflect the extrinsic fibrinolytic activity mainly. We studied the effect of C1.INA using fibrin plate assay on fibrin lysis activity (FLA) and C1.IRFA in normal individuals and pregnant women, and studied the fluctuation of FLA and C1.IRFA during venous occlusion test (VO) or 1-deamino-8-D-arginine vasopressin (DDAVP) administration. After VO or DDAVP administration, the euglobulin lysis time (ELT) was reduced, FLA increased and C1.IRFA also increased. The increase in C1.IRFA accounted for most of the increase in FLA. Therefore, the increased fibrinolytic activity during VO or DDAVP administration is due to extrinsic fibrinolytic activity. In the pregnant women, the fibrinolytic activity was decreased during pregnancy and the low fibrinolytic activity is suggested to be involved in the low extrinsic fibrinolytic activity. The measurement of C1.IRFA is considered to be useful.