AbstractA Burkitt's lymphoma was transferred to the congenitally athymic mouse mutant nude with biopsy material from a 7‐year‐old Kenyan girl. The tumor grows locally at the site of inoculation with no distant metastases. The established tumor has been maintained for six passages so far with preservation of histological and cytological appearance. The mouse‐passaged tumor has a normal human diploid female chromosome complement. Isozyme studies have shown tumor to be of the same glucose‐6‐phosphatedehydrogenase (G‐6‐PD) and phosphoglucomutase1 phenotype (B and 2‐1 respectively) as tumor biopsy from the patient. The mouse‐passaged line maintained surface IgM, similarly to the original tumor and the derived tissue culture line, but lost the IgG coating characteristic for the original tumor but absent from two subsequent biopsies and from the derived tissue culture line. This is in line with previous observations indicating that surface‐associated IgG on Burkitt biopsies is due to coating from the outside, whereas surface‐associated IgM is a cell marker. Whereas the biopsy cells of the patient were positive for EBV‐associated membrane antigen (MA), but not for early antigen (EA) and viral capsid antigen (VCA), the mouse‐passaged line was positive for all three. This suggests that the restrictive influence of the human host on the production of EA and VCA in the Burkitt tumor is raised in the mouse host. The serum of the tumor‐bearing nude mice contained anti‐human antibodies, but no detectable EBV (anti‐MA, EA and VCA)‐antibodies.