The major subtypes of anti-neutrophil cytoplasmic antibodies (ANCA) detected by indirect immunofluorescence assay (IFA) are P-ANCA and C-ANCA. In patients with vasculitis, myeloperoxidase (MPO) is the major P-ANCA antigen and proteinase 3 (PR3) is the major C-ANCA antigen. BPI and azurocidin, which are also called 57-kD cationic antimicrobial protein (CAP 57) and 37-kD cationic antimicrobial protein (CAP 37), respectively, have been proposed as less frequent target antigens for C-ANCA and P-ANCA. In patients with renal disease, we determined the frequency of antibodies against BPI and azurocidin. By IFA on alcohol-fixed neutrophils, monoclonal and polyclonal anti-BPI antibodies produced a C-ANCA pattern, whereas rabbit anti-azurocidin antibody produced a P-ANCA pattern. By ELISA, sera from 229 P-ANCA-positive patients, 99 C-ANCA-positive patients and 48 ANCA-negative (by IFA) patients with renal biopsies were tested for reactivity with recombinant human BPI and purified human azurocidin. Of these sera, 17.5% of P-ANCA, 30.3% of C-ANCA and 20.8% of IFA-ANCA-negative sera were positive for anti-BPI; and 8.3% of P-ANCA, 3.0% of C-ANCA and 8.3% of IFA-ANCA-negative sera were positive for anti-azurocidin. There was no statistical difference in frequency of anti-BPI between pauci-immune necrotizing and crescentic glomerulonephritis (NCGN) and other glomerular disease (OGD), and there was a lower frequency of anti-azurocidin in NCGN samples than in OGD samples. By Western blot, anti-BPI-positive sera reacted with a 57-kD BPI band and anti-azurocidin-positive sera with a 29-kD azurocidin band. In conclusion, there is a low frequency of anti-BPI and anti-azurocidin antibodies in ANCA-positive patient sera; however, this does not correlate with NCGN, which is a marker for ANCA-associated small vessel vasculitis, and a similar positivity is found in IFA-ANCA-negative patients with renal disease. Therefore, serologic detection of anti-BPI and anti-azurocidin is not diagnostically specific in patients with renal disease.