In order to study the genetic mechanisms involved in tumor metastasis, high molecular weight DNA fragments were extracted from human PG tumor cells which were known to be highly metastatic in the nude mice. The DNA fragments were incorporated into NIH/3T3 cells by using calcium phosphate precipitate method. The transfected NIH/3T3 cells were tumorigenic and after a second-round transfection, one of the transfectants (TF87-2-10) was found metastatic to the lungs of nude mice. Human Alu sequences were detected in TF87-2-10 cells by Southern blot analysis. So, it seems that the metastatic capacity of TF87-2-10 cells is associated with the transfection of DNA fragments from PG tumor cells. The relationship between expression of metastatic phenotype of TF87-2-10 cells and host immune mechanisms was studied by injecting TF87-2-10 cells into athymic nude mice and beige nude mice. TF87-2-10 cells showed a higher metastatic rate in beige nude mice (6/13, 46%) than in the athymic nude mice (5/25, 20%). The results suggest that the metastatic expression of TF87-2-10 cells is subject to the influence of host NK cell activities.