» Influenza Virus Vaccine conforms to the regulations of the FDA concerning biologics (see Biologics 1041). It is a sterile, aqueous suspension of suitably inactivated influenza virus types A and B, either individually or combined, or virus sub-units prepared from the extra-embryonic fluid of influenza virus-infected chicken embryo. The strains of influenza virus used in the preparation of this Vaccine are those designated by the U. S. Government's Expert Committee on Influenza and recommended by the Surgeon General of the U. S. Public Health Service. Influenza Virus Vaccine has a composition of such strains and a content of virus antigen of each, designated for the particular season, of not less than the specified weight (in micrograms) of influenza virus hemagglutinin determined in specific radial-immunodiffusion tests relative to the U. S. Reference Influenza Virus Vaccine. It may contain a suitable antimicrobial agent. If formalin is used for inactivation, it contains not more than 0.02 percent of residual free formaldehyde.