Coumarins and structurally related compounds have been recently shown to inhibit replication of human immunodeficiency virus (HIV) and thus, exhibit a therapeutic potential. In this study we report that mesuol and isomesuol, two 4-phenyl coumarins, isolated from the tree Marila pluricostata, suppress HIV-1 replication in Jurkat T cells. These coumarins do not affect the reverse transcription and integration steps of the viral cycle and their antiviral effect is additive with that of azidothymidine (AZT). In addition, mesuol inhibits TNFalpha-induced HIV-1-LTR transcriptional activity by targeting the nuclear factor-kappaB (NF-kappaB) pathway. While mesuol does not prevent either the binding of NF-kappaB to DNA or the phosphorylation and degradation of NF-kappaB inhibitory protein, IkappaBalpha, it inhibits the phosphorylation and the transcriptional activity of the NF-kappaB p65 subunit in TNFalpha-stimulated cells. These results highlight the potential of the NF-kappaB transcription factor as a target for anti-HIV-1 compounds such as 4-phenyl coumarins, which could serve as lead compounds for the development of additional therapeutic approaches against AIDS.