The purpose of the study was to evaluate the involvement of serine proteinases cathepsin G and elastase on pathomechanisms in synovial fluid (SF) of patients with reactive (ReA) and rheumatoid, (RA) arthritis. Cathepsin G, elastase, and their endogenous inhibitors alpha1-antichymotrypsin (alpha1-ACT) and alpha1-proteinase inhibitor (alpha1-PI) were identified immunohistochemically from SF and peripheral blood (PB) of patients with ReA and RA. Cathepsin G and elastase activities in SF and PB were measured spectrophotometrically. Dot-immunostaining was used to identify cathepsin G, elastase, but also alpha1-ACT and alpha1-PI from SF and PB. Cathepsin G and elastase-like activities (IU/I) were slightly elevated in ReA SF compared to the corresponding peripheral blood values (11.4 +/- 9.2 vs 4.8 +/- 1.7, NS, and 5.1 +/- 2.8 vs 2.3 +/- 2.2, NS), which was similar to what was seen in RA (16.4 +/- 6.2 vs 0.53 +/- 0.4, p < 0.05, and 6.51 +/- 1.8 vs 1.22 +/- 0.58, p < 0.05). Although some samples did not contain cathepsin G and/or elastase-like activities, all samples contained immunoreactive enzyme, but also alpha1-ACT and alpha1-PI. In ReA SF, in contrast to monocytes, all polymorphonuclear (PMN) cells contained cathepsin G and elastase. Cathepsin G and elastase activities correlated with each other (r = 0.78, p < 0.05) suggesting PMN / primary granules as their likely source. There was a closer association between the cathepsin G or elastase and SF leukocyte count in ReA than in RA. In ReA and RA SF elevated cathepsin G and elastase activities are detected compared to activity levels in PB suggesting local production mainly from PMNs. The co-existence of highly cellular SF and cathepsin G and elastase activity in the documented presence of endogenous inhibitors in ReA SF together with the, known, usually self-remitting clinical course of ReA, suggest a brisk and even exaggerated local PMN serine proteinase release; sparing of joints does not seem to be due to lack or inhibition of PMN responses but rather to a successful down-regulation or cessation of the responses initially elicited.