Based on the structural analysis of FIV protease and drug-resistant HIV proteases and molecular modeling, a new type of inhibitors with a small P3 residue has been developed. These inhibitors are effective against HIV and its drug-resistant mutants, as well as SIV and FIV. Modification of existing HIV protease inhibitors by reducing the size of the P3 residue has the same effect. This finding provides a new strategy for the development of HIV protease inhibitors effective against the wild-type and drug-resistant mutants. It further supports the use of FIV protease as a useful model for drug-resistant HIV proteases, which often have a more constricted binding region for the P3 group or the combined P3 and P1 groups.