The effects of neonatal thymectomy and antilymphocyte serum (ALS) treatment on the transplantability of a human leukemia were studied in hamsters of varying ages. This tumor, H-HM-1, is normally transplantable only in ALS-treated neonatal hamsters. Multiple doses of ALS prolonged the period of susceptibility to tumor implantation, as compared to a single dose at the time of tumor inoculation. Neonatal thymectomy alone did not substitute for ALS administration, but a combined program of neonatal thymectomy and multiple doses of ALS greatly depressed the host resistance to this tumor xenograft, allowing successful transplantation as late as 20 days after birth. These studies suggest that a thymic-dependent immune mechanism is involved in the rejection of a human tumor xenograft by hamsters.

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These studies were supported in part by Research Grant C-6516 from the National Cancer Institute and FR-05526 from the Division of Research Facilities and Resources, NIH.

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