Features of tissue damage in invasive amoebiasis, in particular polymorphonuclear neutrophil (PMN) degranulation and vasculitis, bear resemblance to that seen in Wegener's granulomatosis, the latter being associated with the presence of anti-neutrophil cytoplasmic antibodies (ANCA). We therefore tested sera from patients with confirmed amoebic liver abscess (ALA) for the presence of ANCA by means of an indirect fluorescent antibody test using pure neutrophils as substrate. ANCA was detected in 97.4% of amoebic sera; the pattern of staining was cytoplasmic, homogeneous, without central accentuation (C-ANCA). A proteinase 3 (PR3) ELISA demonstrated PR3 specificity in 75% of C-ANCA-positive ALA sera. Possible explanations are (i) a cross-reacting antibody to a component of Entamoeba histolytica, or (ii) an antibody to PMN components released, and possibly modified, by the action of E. histolytica on PMN. It is possible that this antibody contributes to the pathogenesis of invasive amoebiasis.