Antineutrophil cytoplasmic antibodies (ANCA) are autoantibodies mainly directed against alpha granules' components (especially proteinase 3 (PR 3) and myeloperoxidase (MPO). They are usually detected by indirect immunofluorescence (IIF) giving essentially two staining patterns, cytoplasmic and perinuclear. Nevertheless the IIF method does not allow to precise the true specificity of ANCA. From now on a better classification of systemic vasculitis requires such a determination. This can be done only by solid phase tests that require to be reliable, highly purified antigen, and, from a practical point of view, only a MPO-ELISA is currently available. We report on our experience with Western blot analysis of 67 IIF-ANCA positive sera. Using Western blot analysis to characterize ANCA specificity is not so easy as in the case of antibodies directed against extractable nuclear antigens: only PR 3 ANCA detection could be done reproducibly. PR 3 ANCA are mainly detected in the c-ACPN positive sera of patients with Wegener's granylomatosis. Nevertheless using both MPO-ELISA and PR 3 blot seems to increase the frequency of serum containing the two types of ANCA (anti PR 3 and anti MPO).