Quantitation of HIV-1 in blood is now widely used by clinicians to manage antiviral therapy. Current methods to detect viral RNA are expensive, have slow turnaround times, and do not directly quantitate infectious particles. Indicator cell assay (ICA) methods for titering HIV-1 rely on the activation of HIV-1 long terminal repeat (LTR)-driven expression of a reporter gene by the viral tat gene product, which is expressed early in the course of infection. The Aequorea victoriana green fluorescent protein (GFP) has proven to be a useful reporter gene for detecting tat-mediated HIV-LTR activation. A general approach to developing a clinically useful ICA required a method of introducing the LTR-GFP expression cassette into various HIV1-infectable cell lines. The LTR-GFP expression cassette was inserted into the LXSN retrovector in a reverse orientation with respect to transcription from the 5' LTR. In cells transduced by the RH5 retrovector, GFP expression was tightly dependent on expression of HIV-1 tat. The PM1 human T-cell line was transduced with RH5 and was further engineered to express the CCR5 HIV-1 CD4 coreceptor constitutively. The resulting cell line, D5-R5, was susceptible to infection by primary HIV-1 strains, macrophage-tropic (M-tropic) and T-cell tropic (T-tropic) laboratory strains, and syncytium-inducing (SI) and and non-SI (NSI) variants. Four days after HIV-1 infection of the indicator cells, GFP expression was detected and quantitated by fluorescence activated cell sorter (FACS), without any false-positive signals. This GFP-based ICA method is of potential use in clinical management of HIV-1, especially in the detection and recovery of drug-resistant virus and the direct determination of antiviral drug sensitivities.