The sera of 64 patients with extracapillary glomerulonephritis were investigated by enzyme-linked immunosorbent assay for the presence of antibody to human neutrophil myeloperoxidase (MPO). In all, circulating anti-MPO were found in 30% and antineutrophil cytoplasm antibodies (ANCA) detected by indirect immunofluorescence in 44% of the patients. Autoantibody to components of neutrophil granulocytes was not found in patients with other forms of glomerulonephritis. The incidence of ANCA (16/23) was higher than that of anti-MPO (5/23) in patients with a diagnosis of Wegener's granulomatosis. By contrast, anti-MPO was found in a majority of vasculitis patients without extrarenal symptoms (6/9), including 3 patients treated with hydralazine. One of the patients treated with hydralazine had circulating ANCA in combination with anti-MPO. Anti-MPO was also found in 1 out of 6 patients with Goodpasture's syndrome. The findings emphasize that autoantibodies to distinct components of neutrophil granulocytes partly differ with regard to diagnostic specificity.