Abstract: A chromogenic assay is described for the identification and isolation of drug-resistant HIV protease mutants. Coversely, the assay is useful to screen for new inhibitors of HIV protease, e.g., inhibitors not affected by drug-resistance of the HIV protease. This color screening assay contains a vector comprising a regulatable promoter which controls the transcription of two adjacent structural sequences, one sequence coding for HIV protease or mutant thereof, the other sequence coding for beta-galactosidase with an amino acid substrate insert cleavable by HIV protease. A library of HIV proteases is also described and is isolated in the form of a collection of such vectors, which is a color screen vector library.