NF-κB is a key regulator of inflammatory gene transcription and is activated in the rheumatoid arthritis (RA) synovium. In resting cells, NF-κB is retained as an inactive cytoplasmic complex by its inhibitor, IκB. Phosphorylation of IκB targets it for proteolytic degradation, thereby releasing NF-κB for nuclear translocation. Recently, two related IκB kinases (IKK-1 and IKK-2) were identified in immortalized cell lines that regulate NF-κB activation by initiating IκB degradation. To determine whether IKK regulates NF-κB in primary cells isolated from a site of human disease, we characterized IKK in cultured fibroblast-like synoviocytes (FLS) isolated from synovium of patients with RA or osteoarthritis. Immunoreactive IKK protein was found to be abundant in both RA and osteoarthritis FLS by Western blot analysis. Northern blot analysis showed that IKK-1 and IKK-2 genes were constitutively expressed in all FLS lines. IKK function in FLS extracts was determined by measuring phosphorylation of recombinant IκB in vitro. IKK activity in both RA and osteoarthritis FLS was strongly induced by TNF-α and IL-1 in a concentration-dependent manner. Activity was significantly increased within 10 min of stimulation and declined to near basal levels within 80 min. Activation of IKK in FLS was accompanied by phosphorylation and degradation of endogenous IκBα as determined by Western blot analysis. Concomitant activation and nuclear translocation of NF-κB was documented by EMSA and immunohistochemistry. Transfection with a dominant negative IKK-2 mutant prevented TNF-α-mediated NF-κB nuclear translocation, whereas a dominant negative IKK-1 mutant had no effect. This is the first demonstration that IKK-2 is a pivotal regulator of NF-κB in primary human cells.