The increase in absorbance was traced during the coagulation of citrated plasma, while in contact with a plastic cuvette surface. The tracings gave rise to coagulation curves, which provided four values. The coagulation curve values were found to change by time due to contact with the surface of the tube containing the plasma. The rate of change was used to determine the intrinsic coagulant activity, in response to a weak activating surface. The value of the determination as a means for detecting hypercoagulability is discussed. The method was also used to study the effect of different factors affecting intrinsic plasma coagulant activity as; the surface of different tubes, plasma concentration, temperature of the tube containing the plasma and platelet concentration.