Three undifferentiated nasopharyngeal carcinoma (NPC) tumour lines were successfully established from fresh biopsy material injected s.c. into athymic mice and passaged for many generations. These xenografts were found to be free of infiltrating lymphoid cells and remained undifferentiated up to passage 30. They were designated NPC/HK2117 (Xeno-1), NPC/HK1915 (Xeno-2) and NPC/HK1530 (Xeno-3), respectively. Passage 16 from Xeno-1, passage 1 from Xeno-2 and passage 14 from Xeno-3 were studied cytogenetically using G-banding with the trypsin-Giemsa method. Two xenografts were hyperdiploid with chromosome modal numbers ranging from 49 to 76, and one was hypodiploid with modal numbers ranging from 28 to 38. Five marker chromosomes have been identified with involvement of chromosomes 1, 3, 11, 12 and 17. Marker chromosomes derived from chromosomes 12q, 1q and 3q were consistent in one of the 2 xenografts successfully G-banded, and chromosomes 12, 11 and 17 were consistent in another. Three out of the 6 markers involve 12q13----qter. An abnormal chromosome 3 with most of the p arm deleted was also observed.