Classic anti-neutrophil cytoplasm antibodies (cANCA), perinuclear ANCA (pANCA) and antibodies directed against myeloperoxidase (MPO-Ab) were evaluated in 25 patients with either idiopathic or secondary rapidly progressive glomerulonephritis (RPGN). While cANCA were found almost exclusively in Wegener's granulomatosis, pANCA were detectable in several disorders, including microscopic polyarteritis (mPA), but also idiopathic RPGN. MPO-Ab were frequently found in sera from patients with all types of idiopathic but not of secondary RPGN. These results support the hypothesis that some cases of RPGN are early or limited forms of systematic vasculitis. We then looked for the presence of IgA-ANCA in Henoch-Schoenlein purpura (HSP): we found IgA-ANCA with immunoenzymatic assay but not with immunofluorescence in HSP, in primary IgA-GN and in membranous GN as well, thus suggesting the poor specificity of this type of ANCA. The possible pathologic implications of ANCA were examined in vitro. Serum samples from several patients with ANCA were assessed for their capacity to enhance chemiluminescence generation from resting or PMA-stimulated macrophages. Sera from RPGN and mPA patients displaying anti-MPO activity induced granulocytes to enhance the production of oxygen free radicals, thus suggesting a phlogistic effect of MPO-Ab positive sera.