A plaque assay system has been developed for types A and B influenza viruses in an established line of canine kidney cells (MDCK-USD). In addition to a homogeneous susceptible cell, consistent plaque production depends on the use of highly purified agar (Agarose). This quantitative system was used to determine the rate of adsorption, synthesis, and thermal inactivation of influenza viruses, as well as to determine a dose response curve. Plaque assays on the MDCK-USD line and the parent MDCK line showed that the latter was more sensitive to A/Swine and A2/Japan 305 viruses. Titration of standard virus pools in embryonated eggs and MDCK-USD indicated that the cell culture system was as sensitive as the in ovo assay.