A plaque assay was developed for feline parvovirus (FPV; feline panleucopaenia virus) in a feline embryo (FEmb) cell line. Higher numbers and larger diameter plaques were obtained with a) seeding rates of 0.7 X 10(5) and 1.5 X 10(5) cells cf. 3 X 10(5) and 6 X 10(5) cells/well of 35 mm diameter, b) synchronised cells infected at the G1-S interface cf. nonsynchronised cells and c) 5 to 6 days incubation post inoculation. The plaque assay was standardised by using serum deprivation for 24 hours to synchronize cells, a seeding rate of 1.5 X 10(5) cells/35 mm diameter well, inoculation of virus 16 hours post seeding followed by 5 days incubation. The standardised assay gave consistent, reproducible results. A dose-response curve using the assay showed a linear, 45 degrees slope, relationship between plaque forming units and virus dilution which further verified the sensitivity and reliability of the assay. Plaques produced by "wild" type and plaque purified virus were invariably non uniform in diameter; diameter of plaques in fact followed a normal frequency distribution under standard assay conditions.