Ig class switch recombination (CSR) is dependent upon the expression of activation-induced deaminase and targeted to specific isotypes by germ-line transcript expression and isotype-specific factors. NF-kappaB plays critical roles in multiple aspects of B cell biology and has been implicated in the mechanism of CSR by in vitro binding assays and altered S/S junctions derived from NF-kappaB p50-deficient mice. However, the pleiotropic contributions of NF-kappaB to gene expression in B cells has made discerning a direct role for NF-kappaB in CSR difficult. We now observe that binding of NF-kappaB components p50 and p65 is detected on Sgamma3 in vivo following lipopolysaccharide (LPS) activation and repressed by LPS + IL-4, suggesting a direct role for this factor in CSR. In vivo footprinting confirms occupancy of a previously defined NF-kappaB recognition site in Sgamma3 with the same temporal kinetics as found in the chromatin immunoprecipitation analysis. Binding of NF-kappaB components p50 and p65 was also detected on Sgamma1 following B cell activation. H3 histone hyper acetylation at Sgamma1 is strongly correlated with NF-kappaB binding, suggesting that NF-kappaB mediates chromatin remodeling in the Sgamma3 and Sgamma1 region.