1. The relative firmness of clots in different systems is not necessarily directly correlated with the speed of clot formation. 2. Thrombin will hasten the development of full clot firmness, but will not significantly alter the degree of clot firmness of citrated plasma specimens, although the firmness of "purified" fibrinogen preparations clotted by thrombin is increased with increasing thrombin concentration. 3. Citrate and/or calcium concentration influence clot firmness. 4. Fibrinogen preparations develop greater firmness on clotting with thrombin than do citrated plasma preparations. 5. A platelet factor influences clot firmness of recalcified plasma, but not of thrombin-activated or coagulase-activated plasma. 6. Clot firmness of recalcified citrated plasma is reduced by storage. This reduction in firmness is more rapid if the specimen is deep frozen as compared to unfrozen aliquots at 6 C.