Objective To study the effect of p50 on IKKα at IL-1β promoter in LPS tolerant cells and to reveal the mechanism of the inhibition of IL-1β mRNA by pS0. Methods THP-1 human promono-cyte model of endotoxin tolerance that simulates the sepsis leukocyte phenotype was used. Chromatin immu-noprecipitation assay(CHIP) and real-time PCR were applied to quantify the binding of p50 and IKKα to IL-1βpromoter. IL-1β mRNA transcription was studied after knocking-down of p50 and/or IKKα. Results With LPS stimulation, p50 binding did not reduce but somewhat increased at IL-1β promoter in tolerant THP-1 cells. Knocking-down of p50 increased the transcription of IL-1β mRNA, which revealed the inhibi-tory effect of p50 in tolerant cells. In contrast, the accumulation of IKKα to IL-1β promoter decreased with LPS stimulation in tolerant cells; However, IKKα binding increased after p50 gene knock-down. In the meantime, IL-1β mRNA transcription increased; At last, IL-1β mRNA decreased again after double-knoc-king down of p50 and IKKα. Conclusion p50 is an inhibitory protein at IL-1β promoter in tolerant THP-1 cells. The unresponsiveness of IL-1β mRNA transcription to LPS at least partly results from the inhibition of IKKα binding to IL-1β promoter by p50.