Proteinase 3, which is known as an intracellular serine protease of neutrophils, was detected at the surface of a subpopulation of freshly isolated PMN. The proportion of PR3-positive and -negative PMN, observed by flow cytometry with anti-PR3 mAbs or ANCA autoantibodies, varies among individuals but is extremely stable for each individual over prolonged time periods. After PMN degranulation by FMLP with cyt. B, membrane PR3 expression increases but the proportion of low and high PR3-expressing cells remains stable. The existence of a subset of PMN which spontaneously expresses PR3 and varies among individuals, may be relevant to the pathogenesis of anti-PR3 ANCA autoantibody-related vasculitis.