The mediator(s) responsible for localized enhanced vascular permeability that characterizes an exacerbation of hereditary angioneurotic oedema (HAE) is thought to be a product of either contact or complement system activation. In contrast to normal individuals, plasma from these patients generates both kinin and vascular permeability-enhancing activity following incubation at 37°C. Depletion of C1 inhibitor in both normal and C2-deficicnt plasma, but not in contact factor-deficient plasmas, resulted in generation of these activities. The kinin activity from incubated HAE plasma was susceptible to kininase inactivation and was blocked by a Bk₂ receptor antagonist. Furthermore, this activity was isolated from HAE plasma; amino acid sequence analysis proved it to be bradykinin. Similarly, the vasopermeability-enhancing activity from ethanol-fractionated or boiled HAH plasma, collected during either attack or remission, co-eluted with bradykinin on reverse-phase high performance liquid chromatography (HPLC). These studies conclusively demonstrate that bradykinin is the major kinin and mediator of enhanced vascular permeability generated during incubation of HAE plasma. The role of other bioactive products, such as the C2 kinin, at local sites of oedema formation remains lo be further defined.