Virus III can be cultivated in vitro indefinitely by serial transfer in simple, minced-rabbit-testis tissue-cultures. The rate of multiplication of the virus rapidly reaches a maximum at which it remains constant. The virus also survives and multiplies readily in rabbit testis tissue-cultures made from the testes of rabbits immunized by previous infection, provided normal, non-immune rabbit serum is used in the preparation, as well as in normal rabbit testis cultures containing the sera of animals of species naturally resistant to the infection. On the other hand, virus III does not survive in tissue-cultures containing the serum of a rabbit immunized by previous infection, nor does it survive in cultures made from the testes of animals of species naturally resistant to the virus, irrespective of the type of serum used.
From these observations on the behavior of virus III cultivated in vitro, it may be concluded in respect to infection by this virus that acquired immunity is humoral in character and depends on the presence of neutralizing antibody. Species immunity, or natural resistance, on the other hand, is cellular in character, and is not related to the presence of an inactivating substance in the blood.