Human vaccines against influenza have been available for almost 60 years and, until recently, were prepared almost entirely from viruses grown in the allantoic cavity of 9- to 11-day-old embryonated chicken eggs. Manufacture involving eggs is not sufficiently flexible to allow vaccine supplies to be rapidly expanded, especially in the face of an impending pandemic. Other problems may arise from the infections of progenitor flocks that adversely affect egg supplies, and from the manufacturing process itself, where breakdowns in sterility can occur from the occasional contamination of large batches of viral allantoic fluid. In addition, egg-grown viruses exhibit differences in antigenicity from viruses isolated in mammalian cell lines from clinical specimens. These concerns and the probable need for greatly expanded manufacturing capability in the future have been brought into focus in recent years by the limited spread of H5N1 avian influenza infections to humans in several Asian countries. Alternative approaches involving the use of accredited anchorage-dependent and -independent preparations of the African Green monkey kidney (Vero), Madin-Darby canine kidney (MDCK) and other cell lines have been pursued by several manufacturers in recent years. Yields comparable with those obtained in embryonated eggs have been achieved. These improvements have occurred in parallel with newer technologies that allow the growth of cells in newer synthetic media that do not contain animal serum, in order to allay the concerns of regulators about the potential for spread of transmissible spongiform encephalopathies.