Glycogen synthase kinase-3 (GSK-3)-α and -β are closely related protein-serine kinases, which act as inhibitory components of Wnt signalling during embryonic development and cell proliferation in adult tissues^,. Insight into the physiological function of GSK-3 has emerged from genetic analysis in Drosophila^,, Dictyostelium and yeast^,. Here we show that disruption of the murine GSK-3β gene results in embryonic lethality caused by severe liver degeneration during mid-gestation, a phenotype consistent with excessive tumour necrosis factor (TNF) toxicity, as observed in mice lacking genes involved in the activation of the transcription factor activation NF-κB. GSK-3β-deficient embryos were rescued by inhibition of TNF using an anti-TNF-α antibody. Fibroblasts from GSK-3β-deficient embryos were hypersensitive to TNF-α and showed reduced NF-κB function. Lithium treatment (which inhibits GSK-3; refs , ) sensitized wild-type fibroblasts to TNF and inhibited transactivation of NF-κB. The early steps leading to NF-κB activation (degradation of I-κB and translocation of NF-κB to the nucleus) were unaffected by the loss of GSK-3β, indicating that NF-κB is regulated by GSK-3β at the level of the transcriptional complex. Thus, GSK-3β facilitates NF-κB function.