Abstract: A clot lysing timing device consisting of a thermostatically controlled assembly including heated blocks formed with wells to receive precooled test tubes containing liquid with clots formed by the addition of thrombin to plasma (or to a mixture of fibrinogen and plasminogen) containing varying quantities of urokinase, streptokinase or plasmin. The blocks are maintained at 37.degree. C. when a precooled test tube with such contents is placed in one of the 37.degree. C. wells it activates a microswitch which starts a corresponding timer channel. An opaque sphere is placed on top of the clot. As the clot dissolves (at a rate depending on the quantity of lytic agent incorporated in the clot) the sphere travels to the bottom of the test tube, interrupting an infra red light beam, which stops the corresponding timer channel. The time counts from the respective timer channels can be selectively displayed on an LED digital readout device.