A method (1 and 2) of producing oncolysis of homologous sarcoma growing in strictly inbred rats, made it possible to obtain rats cured of malignant growths that originated in their own strain and that otherwise would have ultimately caused the death of the animals. Rats treated in this manner proved to be resistant to the growth of transplanted tumors. Further tests, using increasingly larger grafts of sarcoma, showed that they developed an immunity from further growth of the same sarcoma (3). It therefore seemed important to study the nature of the change that takes place in rats, following recovery from sarcomatous growth, which protects them from further tumor growth.

The investigations described in the present report are concerned with the length of sojourn required to inactivate transplanted tumor tissue in immune rats, and with an attempt to ascertain the role of various tissues in this inactivation.

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