OBJECTIVE: To search for anaphylatoxin activity in plasma during episodes of erythema marginatum, and to evaluate the histology of erythema marginatum by electron microscopy and immunohistologic techniques. METHODS: Plasma samples were studied for C5a activity by granulocyte aggregation, and C3 conversion by immunoelectrophoresis. A skin biopsy of erythema marginatum was done, and the tissue stained with a rabbit antibody to bradykinin. RESULTS: No plasma anaphylatoxin was found. Dense deposits of bradykinin were discovered in stromal tissue and lining endothelial cells. CONCLUSIONS: Bradykinin may be important in the causality of the erythema marginatum associated with hereditary angioedema.