Flaujeac trait plasma resembled Hageman trait or Fletcher trait, in that the intrinsic coagulation pathway, plasma fibinolytic pathway, kinin-forming system, permeability factor of dilution (PF/dil) phenomenon were abnormal. The defect in each assay was reconstituted by afactor separable from Hageman factor or Fletcher factor. This substance was an alpha-globulin with an approximate mol wt of 170,000. Flaujeac plasma did not release a kinin upon incubation with kallikrein and was deficient in total kininogen antigen. Antiserum to kininogen inhibited the activity of the factor in solution. Flaufeac factor was identified as a kininogen of high molecular weight (HMW-kininogen). The mean total kininogen antigen in four children of the proposita was 51% (range 34-62%) of normal. A functional coagulation assay of HMW-kininogen in the children was 34% (range 23-55%). The results were consistent with autosomal recessive inheritance. The plasma pathways of intrinsic coagulation, fibrinolysis, kinin formation, and PF/dil generation are dependent upon HMW-kininogen. We believe this is the first demonstration of biological function for a kininogen apart from its role as a substrate for kallikreins.