Human labour is associated with persistently increased nuclear factor kappa B (NF-kB) activity in amnion. In this study we have shown that this involves only the p65 and p50 NF-kB subunits and is associated with an increase in the expression of p65 (P < 0.05), and of the NF-kB binding proteins IkBa, IkBb-1 and IkBb-2 (P < 0.05). Interleukin-1b stimulation leads to rapid degradation and resynthesis of IkBa within 2 h, and a decrease in IkBb-1 without a return to full expression by 2 h, but has little effect upon IkBb-2. IkBb-2 was found in both the cytosolic and nuclear protein fractions. These findings demonstrate that persistently increased NF-kB activity in amnion occurs despite increased expression of the inhibitory IkBa protein and is not mediated by persistant I-kappa kinase activity or inhibition of IkBa synthesis. The increased expression and nuclear localization of IkBb-2 suggests that its function may be to protect NF-kB from inactivation by IkBa and to maintain NF-kB-mediated gene transcription.