A blood clotting end point detector which includes means for monitoring an electrical signal representing the value A of one or more optical properties of a blood plasma sample to which a coagulating agent has been added, means for deriving the first and the second differentials with respect to time of the value A, means for detecting the moment Ta when the second differential changes from negative to positive or becomes positive, means for detecting the moment Tb when the second differential next changes from positive to negative, means for deriving the difference (R) between the tangent to the curve A at the moment Ta and the curve A, means for determining whether the difference (R) or the integral, with respect to time from moment Ta to moment Tb, of the difference (R) exceeds a predetermined threshold value, in which case moment Tb is taken as the true clotting end point.