Basal transcription of the HIV-1 genome is controlled by a variety of ubiquitous and inducible regulatory factors, some with the ability to associate with the viral DNA sequences within the promoter spanning the long terminal repeat (LTR). In this report we demonstrate that activation of the HIV-1 promoter through the inducible DNA binding NF-kappaB transcription factors can be affected by cdk9 in human astrocytic cells. Our results show that ectopic expression of cdk9, but not its mutant variant which lacks the domain responsible for its kinase activity, augments transcription of the LTR. Moreover, we demonstrate that induction of the NF-kappaB pathway by PMA, or overexpression of its subunits including p50/p65 have a negative effect on the ability of cdk9 to stimulate viral gene transcription in these cells. Results from band-shift experiments demonstrated significant suppression of p50/p65 association to its DNA target motif by cdk9. Further, data from GST pull-down and combined immunoprecipitation/Western blot analysis of the protein extracts from cells expressing cdk9, p50 and p65 have revealed the interaction of cdk9 with both p50 and p65 in the absence of DNA containing the kappaB motif. All of these observations led us to conclude that the interaction of cdk9 with the NF-kappaB factors can determine the ability of NF-kappaB to modulate HIV-1 gene transcription.