Nuclear factor-κB (NF-κB) is an inducible transcription factor central in the regulation of expression of a wide variety of genes and synthesis of several proteins involved in the generation of the immune response and inflammatory processes. In resting cells, NF-κB is maintained in an inactive state through cytoplasmic retention by IκB inhibitors. Stimulation of cells with a wide variety of inducers results in proteolytic degradation of these IκB proteins, leading to activation of NF-κB. The present study shows that interleukin-1 (IL-1) causes persistent activation of NF-κB in glial cells. Stimulation with IL-1 also causes rapid but transient degradation of IκB-α and IκB-ε. However, NF-κB remains active even after these IκB isoforms have returned to control levels. In contrast, the IκB-β isoform fails to reappear following its initial degradation by IL-1, coincident with sustained activation of NF-κB. In addition, in vivo overexpression of the various IκB isoforms revealed that IκB-β is the only isoform that has the ability to inhibit IL-1-induced NF-κB-driven transcription. The findings also suggest that the inability of IκB-α and IκB-ε to modulate NF-κB activity is due to their modification in vivo. These findings indicate that IκB-β is the key regulator of the activity of NF-κB in human glial cells.