Abstract
1. In the serum of mix-bred rats that had failed to develop tumors more than 215 days after the start of the feeding test, neutralizing antibodies against the HST virus (agent of the Yoshida tumor) have clearly been demonstrated. With such albino rats, transplantation of the Yoshida tumor was not successful.
2. From the new tumor that was induced by the feeding test a filtrable agent has been isolated by the same procedure as that used in isolating the agent of the Yoshida tumor.
3. The identity of both agents has been established by cross immunization tests: When a rabbit was subjected to active immunization with one of the two agents, neutralizing antibodies against the other agent were produced in the serum. In the case of albino rats of the Wistar strain treated in a similar manner, the same extents of immunity against both tumors were acquired simultaneously.
Between the new and the original tumors a complete cross immunity experimentally comes into being though they morphologically show a different character to some extent.
