Ten successive subcutaneous injections at 2 to 3 day intervals of 0.5 to 1 ml of an alcoholic extract of rat sarcomata was followed by the development of resistance to the growth of transplanted tumor grafts in 50 per cent of the 94 rats treated in 19 litters of rats of the King A strain.

When the tumor-resistant rats were inbred, the percentage of tumor resistance that developed in response to this type of immunization increased, giving a total of 90 per cent, 83 per cent, and 85 per cent in the various inbred generations of the SA, SB and SC lines respectively.

All 173 rats that developed resistance to tumor growth following injections of tumor extract remained resistant to the growth of all grafts of tumor tissue that they received during the several months (6 to 12) they were kept under observation.

The percentage of tumor-immunity increased from 50 per cent of the 94 rats treated in the 19 litters of King A rats selected at random (Table I) to 86.8 per cent of the 145 rats treated in the 31 litters of inbred treated parents (Table II).

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This investigation was supported in part by a research grant from the National Cancer Institute, U. S. Public Health Service.

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