Polymorphonuclear leukocyte (PMN) respiratory burst was stimulated by heterologous antibodies against PMN granule proteins but not by control antibodies. Fluorescence‐activated cell sorter (FACS) analysis of activated PMN demonstrated the presence of two primary granule proteins, proteinase 3 (PR‐3) and cationic protein 57 (CAP‐57) at the membrane surface. The presence of myeloperoxidase (MPO) at the cell surface of primed and unprimed PMN was confirmed by immunoelectron microscopy. Priming doses of recombinant tumor necrosis alpha (rTNFα) enhanced the rate of superoxide (O₂ ^‐) production by these antibodies and increased the amount of surface protein accessible to these antibodies. Anti‐neutrophil cytoplasmic autoantibodies (ANCA) with specificities for PMN granule proteins are present in patients with Wegener's granulomatosis, polyarteritis nodosa, and idiopathic and crescentic glomerulonephritis. The demonstration that antibodies against granule proteins activate PMN supports the hypothesis that the vasculitis seen in these diseases is due in part to PMN mediated oxidative injury following PMN stimulation by ANCA.