Nuclear factor-kappaB (NF-kappaB) and the glucocorticoid receptor (GR) are transcription factors with opposing actions in the modulation of immune/inflammatory responses. NF-kappaB induces the expression of proinflammatory genes, while GR suppresses immune function in part by suppressing expression of the same genes. Previously, we demonstrated that physiological antagonism between NF-kappaB and GR is due to a mutual transcriptional antagonism that requires the p65 subunit of NF-kappaB and multiple domains of GR (1). To elucidate the mechanism(s) of NF-kappaB p65 and GR transcriptional antagonism, we analyzed the interactions of wild-type p65 and p65 RHD (rel homology domain, a dominant negative mutant of p65 which lacks a transactivation domain) with GR. We show that p65RHD blocks p65-mediated transactivation, yet does not block the repression of GR transactivation by p65, indicating that transcriptional activity by p65 is not required to repress GR function. Both p65 and p65 RHD physically interact with GR, but only intact p65 represses GR-mediated signaling, implicating the p65 transactivation domain in the transcriptional repression of GR. To further characterize p65-GR interactions, we examined the role of the transcriptional co-integrator CREB binding protein (CBP) in their mutual antagonism. GR-mediated repression of p65 transactivation and p65-mediated repression of GR transactivation, as well as the physical interaction between NF-kappaB and GR, are enhanced by CBP. GR bound to the antagonist RU 486, although transcriptionally inactive, retains the ability to repress p65 transactivation. However, CBP does not physically interact with antagonist-bound GR and does not enhance its repressive effect on p65. These data suggest that CBP functions as an integrator of p65/GR physical interaction, rather than as a limiting cofactor for which p65 and GR compete.