Physiological inhibitors were tested for their in vitro interaction with neutrophil proteinase 3 (PR3). The major plasma proteinase inhibitor of PR3 is alpha 1AT. We have developed a radioimmunoassay (RIA) for quantitative detection of PR3-alpha 1AT complexes formed in vivo in inflammatory exudates such as synovial fluid and plasma from patients with sepsis. Levels of PR3-alpha 1AT complexes correlated significantly with levels of human neutrophil elastase (HNE)-alpha 1AT complexes. Thus, in vivo alpha 1AT not only protects against excessive HNE activity, but also against excessive PR3 activity.