Mice given injections of living methylcholanthrene-induced fibrosarcoma cells after the complete surgical removal of a growing, sensitizing tumor implant showed a strong immune resistance to the growth of the injected cells. If the sensitizing tumor was left in situ until 4 days after the challenge injection of tumor cells or if the tumor was killed with irradiation and left to regress, the immune resistance of the host was impaired but not abrogated. The depression was specific, since the presence of a growing, antigenically unrelated tumor did not impair the resistance to the sensitizing fibrosarcoma. Injection of graded doses of killed tumor cells of the fibrosarcoma and of the antigenically unrelated tumor, a mammary carcinoma, showed that, at moderate and low doses, only the injection of fibrosarcoma cells depressed host resistance. Large doses of either tumor depressed host resistance, probably in part because of nonspecific toxic depression.

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This work was supported in part by USPHS Grants CA05047, CA6294, CA11430, and CA11138 from the NIH.

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