IFN-γinhibits IL-1β induced MMP-1 and MMP-3 production by RA FLS in vitro. Fibroblast-like synoviocytes (FLS) were isolated from rheumatoid arthritis (RA) synovial tissue specimens. Ten RA FLS cell lines were studied in all. At fourth passage RA FLS were incubated with either culture medium, IL-1β (0.1 ng/ml), escalating doses of IFN-γ (0.1 to 10 ng/ml) or combinations of the two cytokines for 72 hours. Culture supernatants were harvested at 24, 48 and 72 hours. Protease concentrations were measured by ELISA; (A) MMP-1 and (B) MMP-3 (mean ± SEM) values are reported. Tukey's test was used in conjunction with a one-way analysis of variance to analyze differences in protease levels. P values less than 0.05 were considered statistically significant; * P < 0.05, **P < 0.01 and ***P < 0.001.