The coagulation time of the blood is generally prolonged during the acute stage of lobar pneumonia, returning to normal during the period of convalescence. There seems to be a simultaneous increase in the quantity of circulating fibrinogen. The lengthening of the coagulation time is probably due to an increased formation of antithrombin. The source of the increased antithrombin and fibrinogen is probably the liver, and the stimulus to increased production of these two substances is due to the nature of the infecting organism.