F(1) hybrid mice syngeneic or semiallogeneic with respect to the relevant tumor were lethally irradiated and then reconstituted with hemopoietic cells from strain CBAT6T6 mice. After chimerism had been established, the animals were inoculated with solid or ascites tumors. Tumor-host cell hybrids were selected from enzyme-deficient solid tumors by explanting the tumor cell suspension into hypoxanthine-amethopterin-thymidine containing medium. The selection of hybrid cells from ascites tumors was achieved by exploiting the difference between the ascites tumor cells and hybrid cells in their ability to adhere to the surface of culture vessels. T6T6 chromosomal and H-2 antigenic markers served to distinguish between the hemopoietic cells derived from the donor graft and the cells of the host. All solid tumors tested fused with cells of the irradiated host, whereas ascites tumors fused with repopulating cells of hemopoietic origin.