This study was carried out to determine the take rate of malignant lymphomas transplanted subcutaneously in nude mice. Lymphoid tissue from patients with a variety of lymphoproliferative and lymphomatous disorders was employed, but only four (8.3% of total) non-Hodgkin's lymphomas of various types grew in the nude mice. These transplants were from patients with lymphoplasmacytic, lymphoblastic, immunoblastic and centroblastic lymphomas; two of them were serially transferred. High grade malignant lymphomas seem easier to transplant than low grade ones. In each case, the tumor growing in nude mice was similar to the patient's tumor. In one case, metastases were found in mesenteric lymph nodes and lung. Electron microscopic study showed that neoplastic cells were associated with peculiar dark cells the nature of which is discussed. Furthermore, in two transplanted lymphomas, neoplastic cells contained C type virus particles, indicating contamination by the murine xenotropic type C virus.