In this study the IL-6 production was studied by synovial cells isolated from patients with either rheumatoid arthritis (RA) or osteoarthritis (OA). The kinetics of spontaneous IL-6 production differs in both groups. Furthermore, the induction of IL-6 by bacterial lipopolysaccharide (LPS) in synovial cell cultures of RA is much more rapid than in those of OA patients. On the other hand, more PGE2 was detected in culture supernatants from synovial adherent cells of OA than in those of RA patients. We also compared the IL-6 production and the amount of IL-6 mRNA in fragments derived from the areas of synovial tissue showing macroscopic signs of intensive inflammation (area A), with those from relatively intact (area B) synovial sites. In synovial fragments, but not in isolated adherent cells at area A the in vitro IL-6 production starts earlier in RA than in OA. In the area A, significantly more CD14+, CD43+ and HLA-DR+ cells were detected than in the other compartment less involved in local inflammatory events.