Anti-neutrophil cytoplasmic antibodies (ANCA), which are present in Wegener's granulomatosis (WG) and other systemic vasculitis, were detected using the immunoperoxidase method rather than the indirect immunofluorescence method. It was possible to distinguish ANCA patterns (c-ANCA, p-ANCA) by the immunoperoxidase method. When neutrophil smears were treated with 0.4% hydrogen peroxide solution, the endogenous peroxidase activity was sufficiently inhibited. ANCA was also detected, using HL-60 cells (human promyelocytic leukemia) and THP-1 cells (human myelocytic leukemia) as substrates.