The present invention relates, to novel methods and substrates for the propagation of viruses. The invention relates to IFN-deficient substrates and methods for propagating viruses in these unconventional substrates. In particular, the invention relates to methods of propagating viruses in immature embryonated eggs, preferably six- to nine-day-old chicken eggs. The methods of the invention are particularly attractive for growing viruses suitable for use in vaccine and pharmaceutical formulations.