The technique of antibody neutralization has been used to study the plasma of a large group of patients with hypoprothrombinaemia of diverse causes. The associated disorders included obstructive jaundice, acute infective hepatitis and chronic parenchymal liver disease. A number of patients were receiving warfarin and salicylate therapy. Irrespective of the aetiology of the prolonged prothrombin time, patients were demonstrated with an excess of clotting protein over clotting activity for factors II, VII, IX and X. In general, this discrepancy paralleled the prolongation of the prothrombin time.