Anti-neutrophil cytoplasmic antibodies (ANCA) in sera from ulcerative colitis (UC) patients have been described as reacting with proteins in the granules of human neutrophils such as cathepsin G and lactoferrin and with yet unidentified antigens. Here we report the existence of a new member of perinuclear ANCA (P-ANCA) in UC patients. In the previous study, we found that UC patients had a novel P-ANCA against neutrophil 28-kD protein. In this study, we purified the same antigens from HL-60 lysates by using reversed phase high-performance liquid chromatography, and revealed that the 28-kD antigen consisted of two different proteins. The N-terminus amino acids of these proteins are identical with those of high mobility group (HMG) non-histone chromosomal proteins HMG1 and HMG2. Immunoblotting analysis of human neutrophil lysates using rabbit anti-HMG1/2 antisera revealed a single band of 28 kD, and the 28-kD band detected by immunoblotting analysis using patient's serum IgG completely disappeared after preincubation with a mixture of HMG1 and HMG2. Furthermore, rabbit anti-HMG1/2 antisera showed a perinuclear staining pattern in indirect immunofluorescence studies using ethanol-fixed neutrophils. These data demonstrate that HMG1 and HMG2 are novel target antigens of P-ANCA. HMGI and HMG2 are distributed in the nuclei and cytoplasm of eukaryotic cells and act as transcription factors. Their intracellular localization and functions are distinct from those of the previously reported granular antigens of P-ANCA.