The antigen specificity of autoantibodies causing perinuclear staining of granulocytes and monocytes (pANCA) was evaluated by analyzing 3000 sera, which were sent to us for screening of anticytoplasmic antibodies (ACPA, synonym: cANCA, anti-proteinase 3). In 620 sera, perinuclear staining was found. Antigen specificity was investigated by a myeloperoxidase ELISA and indirect immunofluorescence with Hep2 cells specific for antinuclear antibodies (ANA). Only 9.8% of the 620 sera showed reactivity with myeloperoxidase (AMPO), while 85.6% contained ANA which induced a pANCA-like staining. A further 4.6% of the 620 sera were neither ANA nor AMPO positive. Therefore, pANCA in general is only an indication that one should look for AMPO or other antilysosomal autoantibodies, when ANA have been excluded. To investigate the disease specificity of AMPO, we examined sera from patients with several well-defined autoimmune diseases. There were only very few positive results in collagen vascular diseases (3/114) (positive/total), primary systemic vasculitis (1/116) and clinically and histologically proven Wegener's granulomatosis (2/213). On the other hand, AMPO were present in patients with different forms of glomerulonephritis (45/192), especially crescentic glomerulonephritis (CGN) (34/79) without immune deposits in their biopsy specimen (3/30 showed trace deposits of IgM). There were, however, additionally 11 patients with symptoms resembling WG (who were cANCA negative, w/o characteristic WG biopsy), who had no obvious renal symptoms. These findings indicate that AMPO are primarily associated with idiopathic GN, especially CGN. Together with anti-proteinase-3 antibodies and anti-glomerular basement membrane antibodies they are an essential serologic parameter in the diagnosis of unclear systemic diseases with renal involvement.