Tolerance of various degrees of primary subcutaneous grafts of mouse tumors was conferred on rats pretreated with suspensions of lyophilized mouse tumors of the same tumor strains.

Inoculations of transplantable mouse tumors into rats resulted in tumors in 84 per cent of the treated rats. These tumors were all of considerable size (up to 6 × 5 cm.). Some of them regressed at intervals up to 42 days, while some persisted to the death of the animal.

Untreated control rats of the same age and strain were resistant to grafts of the same mouse tumors.

The mouse tumors grown in rats could be transplanted further from rat to rat for several passages, but only in treated rats.

Tumors from different rat passages could be transplanted back into mice of the original strain. Tumors of the seventh rat-to-rat passage were still strain-specific for mice.

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