ANCA have been demonstrated to induce the respiratory burst in primed neutrophils. In this study we have extended the investigations on neutrophil activation by ANCA directed against proteinase 3 (PR3). Myeloperoxidase (MPO) and lactoferrin (LF), and we have analysed the underlying mechanisms. All three ANCA antigens were expressed on the cell surface of primed neutrophils. Superoxide production assayed by both cytochrome c reduction and oxidation of dihydrorhodamine 123, was induced by heterologous polyclonal anti-MPO and anti-LF antibodies, and ANCA-positive plasma samples. Induction of superoxide production was dose-dependent. F(ab)2 fragments did not induce the respiratory burst. Blockade of Fc receptors by specific oAbs showed that anti-FcγRII antibodies were able to turn off the ANCA-induced respiratory burst, whereas anti-FcγRII antibodies did not. Plasma samples that induced the respiratory bursl did not differ from samples that did not induce superoxide production with respect to ANCA titre, but had higher levels of the lgG3 subclass of ANCA. Levels of the other subclasses of ANCA were comparable between those samples. We conclude that ANCA-induced activation of primed neutrophils is FcγRII-dependent, and appears to be facilitated by antibodies of the IgG3 subclass.