1. Shielding one Peyer's patch of a mouse during lethal x-irradiation protects a sufficient number of cells to cause rejection of a heterologous or homologous bone marrow graft. 2. Shielding 11 mm of the small intestine results in a similar but less violent reaction. 3. A dosage of 650 r or more to the protected Peyer's patch is necessary to impair the capacity of the lymphatic tissue to reject the graft. 4. Regeneration of lymphatic tissue is evident in the spleen, mesenteric lymph node and thymus in Peyer's patch-shielded mice 6 days post-irradiation. On the other hand no bone marrow regeneration was observed nor was there evidence of granulocytic, megakaryocyte, or erythrocytic regeneration elsewhere.