ABSTRACT. Three feline coronavirus (FCoV) isolates KUK-H, M91-266, and M91-267 were examined to elucidate their biological and antigenic properties as well as disease potential in cats. Immune stainings of virus-infected cells by using FCoV type-specific monoclonal antibodies indicated that their antigenic specificity was serotype II. However, antigenic variations among these serotype II FCoVs were detected by neutralization assay with hyperimmune antisera against FCoVs and canine coronaviruses, and with experimentally infected cat sera; there were two subtypes in serotype II FCoVs. The isolates efficiently grew in fcwf-4 cell culture showing lytic CPE enough to form distinct plaques; when measured 48 hr after infection, plaque sizes of both M91-266 and M91-267 were approximately 1 mm in diameter, and a mixture of small (less than 1mm in diameter) and large (approximately 3 mm in diameter) plaques were produced in the case of KUK-H. Strains KUK-H, M91-266 and M91-267 produced feline infectious peritonitis (FIP) in 50%, 67% and 89% of experimentally inoculated kittens, respectively. Furthermore, 80% of the kittens inoculated with the small plaque former of KUK-H developed FIP accompanied by more prominent clinical signs as well as pathological changes when compared with 28.6% of kittens inoculated with the large plaque former. These results suggest that serotype II FIPVs producing smaller size of plaques are more virulent than those producing larger size of plaques. -KEY WORDS: coronavirus, feline, feline infectious peritonitis, plaque, virulence.J. Vet. Med. Sci. 59(4): 253-258, 1997 serotype II FCoV has arisen by recombination between serotype I FCoV and CCV. In the present paper, three serotype II FCoV isolates from clinical FIP cases were examined in respects of antigenic property and in vivo pathogenic virulence especially in relation to plaque characteristics. Discussion was also made regarding relationships between plaque properties and virulence of coronaviruses in animals. MATERIALS AND METHODSViruses and cell culture: KUK-H strain was isolated in 1987 by using CRFK cells (ATCC CCL94), and both M91-266 and M91-267 strains were isolated in 1991 by using fcwf-4 cells. All derived from the spleen samples taken at the postmortem examination of effusive form FIP field cases. Stock viruses were prepared after further several passages in fcwf-4 cell culture. Strain C3663, a local isolate from an effusive form FIP case, was used as a reference serotype I FCoV. The cells were cultured as described previously [18].Plaque assay: The stock viruses were characterized by plaque assay, and then the virus was plaque-purified for further experiments. After serial tenfold virus dilutions, 0.2 ml of each dilution was inoculated onto fcwf-4 cell monolayer in 60-mm plastic plates. The plates were incubated at 37°C for 1 hr, and 4 ml of an overlay medium was added. The plates were further incubated at 37°C for 2 or 3 days and then stained with either an overlay medium According to the recent concept about feline coronaviru...