The tissue from a patient with nasopharyngeal carcinoma has been transferred to nude mice (BALB/c), and has successfully been growing through twenty passages. The tumors in the nude mice and primary human tumor have been examined for the cellular DNA content by FCM and also conventional pathological examination, chromosome analysis, and EBV test. The tumor take rate varied markedly in different passages with a mean value of approximately 70%, and showing a tendency to increase. The tumor doubling time within 6-12 weeks after transplantation of six tumors in 18 and 20 passages were 14.8 and 9.3 days respectively. However, the tumor volume at 12 weeks varied significantly, ranging from 438 to 1998 mm3. By FCM, it has been found that the values of DNA index were about the same in both primary tumor and the tumors in nude mice. The distribution of various phase cells in cell cycle was also about the same in both. In conclusion, the application of FCM to examine the cellular DNA content of the tumor in nude mice is a rapid and sensitive method, useful in the investigation on the stability of biological characteristics of human NPC in nude mice and in further studies on the effects by various therapeutic methods.