['To establish whether the steroid hormone retinoic acid (RA) was able to modulate the activity of the HIV-1 promoter.', 'The effect of RA and nuclear factor (NF)-kappa B on HIV-1 promoter activity was investigated using HIV-1 long terminal repeat (LTR)-based reporter constructs.', 'The activity of wild type and mutant LTR sequences was compared in a variety of stably and transiently transfected human cell lines.', 'It was shown that RA treatment inhibits both the basal activity of the HIV-1 LTR and its stimulation by phorbol ester treatment. This inhibition is observed in both HeLa cells (in the presence of exogenously supplied RA receptors) and the naturally RA-responsive U937 monocyte cell line. RA can also inhibit the stimulation of the HIV-1 LTR by overexpression of NF-kappa B subunits, while linkage of the NF-kappa B sites in the HIV promoter to a heterologous promoter results in its inhibition by RA.'] Does retinoic acid inhibit both the basal activity and phorbol ester-mediated activation of the HIV long terminal repeat promoter?