Anti-endothelial cell antibodies (AECA) have been detected by cell surface radioimmunoassay innine out of 15 patients with micropolyarteritis (MPA) and in two out of five patients with Wegener'sgranulomatosis. AECA mostly belonged to the IgG isotype and were present in the active phase ofthe diseases. These antibodies were not detectable in 10 sera from patients with essential mixedcryoglobulinaemia. suggesting that they were not a mere epiphenomenon consequent to theinflammatory vascular injury. The binding activity was not related to ABH antigens or to HLA class Iantigens displayed by resting human endothelial cells in culture and was not influenced by removingimmune complexes. Absorption of the anti-neutrophil cytoplasmic antibodies (ANCA). present inMPA and Wegener's granulomatosis sera, did not affect the endothelial binding. AECA-positive seradid not display lytic activity against endothelial cells, neither alone nor after addition of freshcomplement or normal human peripheral blood mononuelear cells. Although AECA arc notcytolytic for endothelial cell monolayers in vitro, the reactivity against intact endothelial cells suggeststheir possible involvement in in vivo pathological processes affecting vascular structures in smallvessel primary vasculitides.