Abstract: Combinatorial libraries of HIV and FIV protease inhibitors are characterized by ?-keto amide or hydroxyethylamine core structures flanked by on one side by substituted pyrrolidines, piperidines, or azasugars and on the other side by phenylalanine, tyrosine, or substituted tyrosines. The libraries are synthesized via a one step coupling reaction. Highly efficacious drug candidates are identified by screening the libraries for binding and inhibitory activity against both HIV and FIV protease. Drug candidates displaying clinically useful activity against both HIV and FIV protease are identified as being potentially resistive against a loss of inhibitory activity due to development of resistant strains of HIV.