Bright/ARID3A is a nuclear matrix-associated transcription factor that stimulates immunoglobulin heavy chain (IgH) expression and Cyclin E1/E2F-dependent cell cycle progression. Bright positively activates IgH transcriptional initiation by binding to ATC-rich P sites within nuclear matrix attachment regions (MARs) flanking the IgH intronic enhancer (Emu). Over-expression of Bright in cultured B cells was shown to correlate with DNase hypersensitivity of Emu. We report here further efforts to analyze Bright-mediated Emu enhancer activation within the physiological constraints of chromatin. A system was established in which VH promoter-driven in vitro transcription on chromatin- reconstituted templates was responsive to Emu. Bright assisted in blocking the general repression caused by nucleosome assembly but was incapable of stimulating transcription from prebound nucleosome arrays. In vitro transcriptional derepression by Bright was enhanced on templates in which Emu is flanked by MARs and was inhibited by competition with high affinity Bright binding (P2) sites. DNase hypersensitivity of chromatin-reconstituted Emu was increased when prepackaged with B cell nuclear extract supplemented with Bright. These results identify Bright as a contributor to accessibility of the IgH enhancer.