Portions of 162 naturally occurring neoplasms and 26 nonneoplastic lesions from 93 aged male or female F344/NCr rats were implanted into the left inguinal mammary fat pads of weanling syngeneic recipients. As controls, 95 normal tissues were implanted to the right inguinal fat pad. Transplant recipients were maintained for up to 1 year. Essentially, all types of naturally occurring benign and malignant tumors were successfully transplanted, i.e., grew progressively forming nodules and masses. For the transplants, the latency period preceding palpable growth, tumor growth rate, invasiveness, metastatic rate, and time to death were associated with the degree of histological malignancy of the primary tumor. The tumors which were the most malignant based on these criteria included large granular lymphocyte leukemia, sarcomas, and carcinomas. Fibromas, mammary fibroadenomas, and papillomas were easily transplanted but were not invasive. Endocrine tumors generally were the slowest-growing tumors. This study provides evidence that successful tumor transplantation is only evidence of neoplasia and does not distinguish whether a primary tumor is benign or malignant.