OBJECTIVE—To study the effect of cytokines on the transactivation of the c-fos gene in relation to the contribution of overexpression of c-fos/AP-1 in rheumatoid joint destruction. METHODS—The promoter region (−447 to +109) of the human c-fos gene was integrated upstream of the chloramphenicol acetyltransferase (CAT) reporter gene, and the effect of cytokines on the expression of the c-fos gene was studied in the rheumatoid synovial cells of early (3-4) or late (14-18) passages, in the presence or absence of cytokines, by the transient transfection assay. RESULTS—Expression of c-fos gene was enhanced by tumour necrosis factor α (TNFα) and interleukin 6 (IL6) in the synovial cells of early passage, whereas it was not enhanced in the synovial cells of late passage. The c-fos gene expression was also enhanced by 13-O-tetradecanoyl phorbol-13-acetate (TPA) in early passage but was somewhat suppressed in the late passage. It was found that the c-fos gene and c-Fos protein were both increased in the synovial cells of late passage. Similarly, c-fos gene expression was also not increased by TPA or cytokine stimulation in the stable c-fos transformants (fos-pH8) or H-ras transformed NIH3T3 cells (NIH H-ras cells) that constitutively expressed c-fos genes. CONCLUSIONS—Although TNFα and IL6 augmented c-fos gene expression of rheumatoid synovial cells, transactivation of c-fos gene became resistant against cytokine stimulation under prolonged expression of c-fos gene, which may impart a tumour-like characteristic to rheumatoid synovial cells.