The human immunodeficiency virus type 1 (HIV-1) contains regulatory regions in its long terminal repeat (LTR) implicated in the control of viral gene expression. We previously demonstrated that Dialyzable Leucocyte Extract (DLE), a preparation derived from immune leukocytes, is able to inhibit HIV-1 replication in MT-4 cell cultures. Here, we examined the effect of DLE on the activation of NF-kappa B and Sp1 transcription factors. NF-kappa B activity was completely suppressed after seven days of treatment with 2.5 U/mL of DLE, with a parallel large reduction in the amounts of Sp1 complexes. These findings correlate with the maximum inhibitory effect on HIV-1 replication described in a previous report. I kappa B alpha and NF-kappa B p65(RelA) gene expression are not regulated by DLE in MT-4 cells. Although up to day, the precise molecular mechanism of DLE biological activity in HIV-1 infection remains unclear, this report presents data that indicate a potential downregulatory effect of DLE on HIV-1 gene expression.