Abstract: Feline Infectious Peritonitis (FIP) is a lethal systematic disease caused by FIP Virus (FIPV). FIPV is divided into two serotypes which differ in their growth characteristics in tissue cultures; serotype II viruses replicate efficiently, whereas viruses in serotype I do not replicate or produce plaques. Here, the effects of proteases on plaque formation by serotype I viruses were examined. The incorporation of trypsin or pronase in an agar overlay medium of Felis catus whole fetus (Fcwf-4) cells had substantially no effect on plaque formation by serotype I viruses. However, addition of diethylaminoethyl (DEAE)-dextran to trypsin-or pronase-containing overlay greatly enhanced plaque formation by serotype I strains. In addition, serotype I viruses from clinical specimens could produce plaques only with an overlay containing both pronase and DEAE-dextran. These findings suggested that both pronase and DEAE-dextran are required for optimal plaque formation by serotype I viruses.