Feline panleukopenia (FPL) virus was isolated on direct monolayer culturefrom the spleen of a cat which had manifested the symptoms of FPL. It formed intra-nucleaf inclusions in cat kidney cell cultures, whereas no definite cytopathogenic effect wasobserved on these cultures.The virus showed a resistance to ether, chloroform, sodium deoxycholate, and pH 3.0.It was not inactivated completely after heating at 80C for 30 minutes. Viral replicationwas inhibited by 5-iodo-2-deoxyuridine. The virus passed through a membrane filter 25 nmin pc)re size. No significant hemagglutination of the virus was observed with the erythro-cyte of guinea pig, rat, rabbit, cat, chicken, sheep, calf or horse, but the virus hemagglu-tinated swine erythrocytes under an appropriate condition. The virus was neutralized onlyby FPL-immune serum among 16 kinds of immune sera against feline viruses. A survey onthe serological evidence of the virus in healthy cats indicated that 50% of the kittens 0 to3 weeks old and all the adult cats examined had neutralizing antibody titers of 1:1024 orover, and that all the 18 kittens 4 to 24 weeks old surveyed had titers of less than l:16.