Anti-neutrophil cytoplasmic autoantibodies (ANCA) associated with active Wegener's granulomatosis are directed against a soluble 29-Kd protein present in human neutrophils and monocytes. Affinity labeling with tritiated diisopropylfluorophosphate (3H-DFP) suggested that ANCA- antigen is a serine protease. We used immunoelectron microscopy to study the in situ localization of the ANCA-antigen in normal human neutrophils and monocytes using immunoglobulin G (IgG) from ANCA- positive patients and a mouse monoclonal antibody against the ANCA- antigen. Label was observed on the large granules of the neutrophils and in granules of monocytes. Double-labeling, using anti- myeloperoxidase or the peroxidase reaction as markers for azurophil granules and anti-lactoferrin as marker for specific granules, showed that ANCA is colocalized with markers of azurophil granules but not with lactoferrin. Furthermore, elastase and cathepsin G were found in the azurophil granules of neutrophils and in the peroxidase-positive granules of monocytes, colocalized with ANCA-antigen. Cytochalasin-B- treated neutrophils stimulated with N-formyl-methionyl-leucyl- phenylalanine (fMLP) formed large intracellular vacuoles and were partially degranulated. Some vacuoles contained ANCA-antigen, as well as myeloperoxidase, elastase, and cathepsin G, demonstrating release of these enzymes from the azurophil granules into vacuoles. Our results demonstrate that ANCA-antigen is located in myeloperoxidase-containing granules of neutrophils and monocytes, and is packaged in the same granules as elastase and cathepsin G, the two previously identified serine proteases of myeloid leukocytes.