Abstract: An in vitro method of monitoring thrombolysis of an occlusive thrombus formed under conditions which simulate in vivo is described. Thrombus is formed in a capillary tube by causing blood to flow under pressure from a non-anticoagulated supply thereof through the capillary lumen and into a reservoir in such a way that the thrombus forms solely by shear-stress. Formation of the thrombus is detected and the pressure on the tube is reduced, the reduced pressure being maintained for a predetermined period of time during which the thrombus stabilises. The pressure is then applied on the thrombus across anticoagulated blood from the same sample and lysis of the thrombus is detected. Preferably the supply of anticoagulated blood is produced by mixing the blood exiting the capillary tube with anticoagulant in the reservoir, the blood flow direction being reversed once thrombus stabilisation has taken place. The reservoir may be connected to a second reservoir by a connecting line.