Summary. Methods have been devised for the measurement of antithrombin and of the natural inhibitor of activated factor X. The relation between heparin and its plasma cofactor and the destruction of thrombin and activated factor X were studied. As observed by other workers, heparin has more effect on the speed of thrombin neutralized than on the amount neutralized after 1 hr of incubation. When the neutralization of activated factor X was studied it was found that heparin at very low concentrations, with a plasma cofactor, promoted the progressive destruction of activated factor X.

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Summary. The inhibitor‐neutralizing ability of plasma and Factor‐VIII concentrates from a group of 48 haemophilic patients and 30 normal persons has been studied. Factor‐VIII inhibitors were neutralized by the material from four of the haemophilic patients and all the normal subjects. The material from the larger group of 44 haemophilic patients showed no significant inhibitor‐neutralizing ability. The former group have been designated Haemophilia A+ because of the presence of material antigenically related to Factor VIII, and the latter group Haemophilia A− because of the lack of such material.