The assay described in this paper is based on the expression of the PR/Gal4 fusion in an inducible manner via a Tet-On system (adapted from Clontech), thus drastically reducing the possible toxic side-effects of PR. The reverse tetracycline transactivator (rtTA) used in this system allows for the induction of PR/Gal4 expression by the addition of tetracycline (Tet) or doxycycline (Dox), only when needed. eGFP, the sensor for PR activity, will be expressed only when PR/Gal4 expression is induced in the presence of a PI. Most importantly, all the assay elements have been stably expressed in mammalian cells using retroviral technology. Finally, we established T-cell lines from clones with the highest sensitivity showing robust, reliable and reproducible behavior. This is the first time a cell-based assay for monitoring PR activity has been developed in T-cells, the natural milieu of HIV infection, thus providing an efficient way to search for novel inhibitors/competitors of PR, which could lead to the development of new therapeutics against HIV.