Two transplantable hamster lymphomas, one of which (ML) metastasizes, while the other (NML) only grows at the site of implantation, were examined in the electron microscope. The tumour cells of the two lymphomas were found to be identical except for a slightly greater amount of ER on ML; both contain the typical ultrastructural features of experimental tumour cells, including viral particles. The difference between the two tumours was found to be in the invading host macrophages. The macrophages in NML are large cells containing phagosomes filled with tumour cells in varying stages of digestion. ML also contains macrophages but these are small cells without phagosomes, which accounts for their not being found in earlier light microscope studies. That these latter cells are macrophages is evident from their membrane processes and the presence of primary lysosomes. It is therefore concluded that the difference between these two lymphomas lies not in the ability of the host macrophages to invade ML, but rather in the inability of the latter macrophages to become activated.