1. In a search for suitable tissues for the cultivation of yellow fever virus in vitro, mouse embryos were inoculated with this virus in utero. A titration for virus content of the various organs of the embryos indicated that the virus was present in the brain in greatest concentration.
2. Unmodified strains of yellow fever virus were readily adapted to cultivation in vitro in a medium consisting of minced mouse embryo brain tissue and Tyrode solution containing 10 per cent normal monkey serum.
3. After a continued cultivation in mouse embryo brain tissue cultures for twenty to twenty-five subcultures, these strains were readily adapted to cultivation in whole mouse embryo tissue medium.
4. There is evidence to indicate that a prolonged cultivation of the virus in mouse embryo brain medium increases its neurotropic properties.
5. Attempts to employ monkey tissues for in vitro cultivation of yellow fever virus gave entirely negative results.