Strain dependent growth of a human carcinoma was observed in nude mice with different genetic backgrounds. Rapid growth of the human gastric carcinoma was seen in CBA/N-nu, NFS/N-nu, and NIH(s)-nu. The human carcinoma in NIH(s)-nu showed the widest deviation in tumor weight. Natural killer activity of spleen cells from nude mice with different genetic backgrounds was not necessarily correlated with in vivo tumor growth. These data suggest that there are complex regulatory mechanisms in the antitumor system.