A novel plaque assay system for paramyxoviruses

A combination of 5% allantoic fluid and 200 /gmg/ml DEAE-dextran with 30 mM MgCl2 used as a supplement to normal overlay medium was found to give large, clear plaques on monolayers of secondary chick embryo fibroblasts infected with avirulent strains of Newcastle disease virus which did not produce plaques without these additions. This modified overlay also allowed plaque assay of Sendai virus strain Z, and avoided any requirement for special cell lines or the use of serumfree medium