Abstract
The production of focal necrosis in hamster kidney cells maintained beneath an overlay of semisolid agar medium free of serum permits rapid, reproducible, quantitative assay for the three subtypes of influenza A viruses. Adaptation of virus to these cells was not required. A straight-line dose-response relationship was observed between the number of plaques produced and the relative concentration of virus. Specificity was demonstrated by quantitative neutralization with immune serum.