IFN-γ does not affect tissue protective TIMP-1 secretion by RA FLS in vitro. Fibroblast-like synoviocytes (FLS) were cultured from synovial tissue specimens obtained from rheumatoid arthritis (RA) patients at synovectomy or joint replacement. Eight cell lines were used at fourth passage. Cells were incubated in culture medium or in medium containing escalating doses of IFN-γ (0.1 to 10 ng/ml), IL-1β (0.1 ng/ml) or combinations of the two cytokines for 72 hours. TIMP-1 levels were quantified by ELISA in culture supernatants at 24, 48 and 72 hours; mean ± SEM TIMP-1 concentrations are reported. (A) TIMP-1 levels were significantly increased by IL-1β but not escalating concentrations of IFN-γ. IFN-γ did not affect IL-1β induced TIMP-1 production by RA FLS; (B) represents data obtained for 10 ng/ml IFN-γ ± IL-1β (0.1 ng/ml). Tukey's test was used in conjunction with a one-way analysis of variance to analyze differences in TIMP-1 levels. P values less than 0.05 were considered statistically significant. NS denotes non-significant change, *P < 0.05 and **P < 0.005.