The target antigen of anti-neutrophil cytoplasm antibodies (ACPA; also known as ANCA) was isolated by affinity chromatography from supernatants of human neutrophils, stimulated with phorbol ester to induce degranulation. Sequence analysis of the antigen revealed 17 NH2-terminal amino acids (IVGGHEAQPHIRPIYMA), which have considerable sequence homology with known serine proteinases. Investigation of the enzymatic activity showed that the antigen is a neutral serine proteinase that is able to cleave elastin. Since the molecular weight of the antigen, its substrate specificity, and its inhibitor profile reported in this study are identical with those reported recently for proteinase 3, we conclude that ACPA are most probably directed against proteinase 3.