1. Females rendered immune actively and passively against yellow fever virus may transmit protective antibodies to their young ones. 2. Mice acquire immunity through the placenta and principally after birth by ingestion of milk of an immune female. 3. The virus can not be transmitted by either of the two ways. 4. The immunity transferred through the placenta disappears much more rapidly than immunity transmited by milk. 5. Young ones from a normal female acquire immunity rapidly if fed by an immune female. 6. Mice of more than 10 days absorb antibodies just as well as during the first days of their birth, and the milk carries with it protective substances during the whole suckling period.