Newborn rats were inoculated with cell suspension of EB-2 or EB-3 Burkitt lines by Southam of the Sloan Kettering Institute, New York. The kidney tumors of the inoculated animals were studied at the electron microscope showing ultra-structural features similar to the original Burkitt tumor and to the several African lymphoma cell lines established in vitro. Two types of cells were described: type I closely resembles the Burkitt lymphoblasts, type II has the morphology of undifferentiated blasts. This cellular variability and the similarity of the II cell type to phytohaemagglutinin transformed lymphocytes are discussed. The absence of herpes-like particles in this material might be related to the lack of cell degeneration.