A novel, highly quantitative transient expression assay based on the human Interleukin-2 (IL-2) gene was used to examine the frans-activation of the Human Immunodeficiency Virus (HlV/HTLV-lII/LAV/ARV) long terminal repeat (LTR) in a range of eukaryotic cell lines. In the absence of the frans-activating viral gene product, faf-Ill, IL-2 transcripts specific for the HIV LTR were present in low abundance in transfected cells and showed a low translational efficiency, when compared with IL-2 mRNAs transcribed from other viral promoters. Coexpression of fat-Ill resulted in a marked increase in the steady state level of IL-2 mRNAs transcribed from the HIV LTR, and these mRNAs also demonstrated a specific enhancement of their translational efficiency. These results suggest a bimodal mechanism of action for fat-Ill in the frans-activation of HIV-specific gene expression.