Feline panleukopenia virus was isolated from a peracute, fatal disease in a 3-month-old Burmese kitten by inoculation of a 1 per cent spleen suspension onto freshly seeded cultures of a feline embryo (FEmb) cell line. The virus was assayed by the detection of intranuclear inclusion bodies in stained coverslips of FEmb cells. The virus was not inactivated by ether, pH 3 or 56° C for 2 hours. The titer of the virus was reduced from 106.7 to 104.2 50 per cent tissue culture infective doses when grown in the presence of 20 μg/ml 2-bromo 5-deoxyuridine. The identity of the isolated virus as a strain of panleukopenia was established by showing that it was antigenically related to a standard vaccine strain of the virus. In negatively stained preparations the virus was polyhedral in shape, with a diameter of 20 to 24 nm; smaller, doughnut-shaped structures 9.5 to 12 nm in diameter were present in most preparations. The maximum buoyant density of the virus in CsCl was 1.4 g/ml though infectivity was associated with lower densities as well. In the properties studied the virus resembles other autonomously replicating parvoviruses.