Provided is a method for treatment and/or prophylaxis of FIP (feline infectious peritonitis) in cats. The method is performed by administering to a cat a composition comprising a therapeutic and/or prophylactic amount of a cysteine protease inhibitor. The cysteine protease inhibitor can be a selective cathepsin B inhibitor. Also provided is a method for inhibiting FIPV (Feline infectious peritonitis virus) replication. The method comprises contacting a cell infected with FIPV with a cysteine protease inhibitor in an amount effective to inhibit replication of FIPV. It is expected the method will be effective for therapy or prophylaxis of FIP in any species of cat and will be effective against any strain of feline coronavirus.