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Yellow Fever Vaccine conforms to the regulations of the FDA concerning biologics (see Biologics 
1041
). It is the attenuated strain that has been tested in monkeys for viscerotropism, immunogenicity, and neurotropism of living yellow fever virus selected for high antigenic activity and safety. It is prepared by the culturing of the virus in the living embryos of chicken eggs, from which a suspension is prepared, processed with aseptic precautions, and finally dried from the frozen state. It meets the requirements of the specific mouse potency test in titer of mouse LD50 (quantity of virus estimated to produce fatal specific encephalitis in 50% of the mice) or the requirements for plaque-forming units in a suitable cell culture system, such as a Vero cell system for which the relationship between mouse LD50 and plaque-forming units has been established, in which cell monolayers in 35 mm petri dishes are inoculated for a specified time with dilutions of Vaccine, after which the dilutions are replaced with 0.5% agarose-containing medium. Following adsorption and incubation for five days an overlay is added of the 0.5% agarose medium containing 1:50,000 neutral red and the plaques are counted on the sixth day following inoculation. It is sterile, and contains no human serum and no antimicrobial agent.
but never above 5, throughout the dating period. Preserve it during shipment in a suitable container adequately packed in solid carbon dioxide, or provided with other means of refrigeration, so as to ensure a temperature constantly below 0.
20, 1 year).