A patient with a lymphoproliferative disorder, angioedema and an acquired deficiency of the inhibitor of the activated 1st component of complement [C.hivin.1] was studied. The patient''s C profile revealed depletion of the 1st component of complement [C1] which was not previously seen in angioedema of the hereditary type. There was no evidence for C1-depleting activity in the patient''s plasma. The majority of the patient''s peripheral blood mononuclear cells resembled B [bone marrow-derived] cells in their membrane receptor properties, and they carried easily detectable immunoglobulin [Ig], predominantly IgM. These cells were unusual in that they phagocytosed latex particles and C3-coated erythrocytes. Morphological study of the cells infiltrating the patient''s lung revealed immature, atypical and plasmacytoid lymphocytes and immunoblasts. The patient''s peripheral blood mononuclear cells and a suspension of cells from the pulmonary infiltrate were capable of depleting C1 and its inhibitor from homologous plasma. Normal ABO-compatible cells did not possess this property. The patient''s abnormal lymphoid cells may have interacted with the C system to produce a biochemical defect and a clinical syndrome closely resembling angioedema of the hereditary type.