Lymphosarcoma-bearing spleen fragments or cell-free extracts from an amphibian (Xenopus laevis laevis) implanted or injected into cockroaches (Periplaneta americana) elicited a characteristic hemocytic reaction. Back-transfer of the abnormal insect tissue to immature Xenopus up to 125 days after implantation led to the development of lymphoid tumors in the toads, thus indicating retention of infectivity during passage through the insect.