The physical principles of a self-heated thermistor used to detect the time at which plasma forms a clot have been developed. A gently vibrated self-heated thermistor is very sensitive to bulk fluid flow. When placed in human plasma which is coagulated by the addition of thromboplastin and calcium, a very distinct signal is invariably obtained, coincident with clot formation. We have shown that the end point is not calorimetric in origin nor is it due to a change in the thermal conductivity of the media. The observed signal is due to a “freezing-out” of fluid flow by clot formation. Optimum resuits in normal plasma (i.e., a fibrinogin level of 200-400 mg/dl) are obtained with small thermistors (radius 0.05-0.13 cm) which are vibrated at 60 Hz with an amplitude of 0.05 cm. Various diffusion models have been tested, and it appears that the time dependence of the phenomenon is functionally similar to slow spherical diffusion in a quiescent fluid. The results have been verified by measuring the thermal diffusivity of several organic solvents using water as a calibrant.