We developed a sensitive vector system for the analysis of weak promoter activities. This promoter assay is based on the transcriptional activator protein, Tat, of human immunodeficiency virus type 1 (HIV-1). High-level expression of HIV requires activation in trans by Tat of the promoter in the long terminal repeat (LTR). Here we describe the construction of a promoterless pTat vector. Foreign promoter elements can be inserted upstream from the tat gene, and expression of Tat protein is measured in trans on a co-transfected LTR-CAT reporter plasmid. We show that this binary system is more sensitive than standard pCAT reporter assays.