Mortality caused by inoculation of cells from four Marek's disease herpes virus-induced transplantable lymphomas was studied in two related inbred lines of chickens. While i.m. inoculation of 10(4) cells from each lymphoma generally caused death of all syngeneic recipients by 18 days post-inoculation, one of the lymphomas (UG2) was unique in that the same number of cells, when inoculated i.v., caused less than 20% mortality by that time. Lethality induced by cells from the other three lymphomas, when inoculated i.v., was as high or higher than when inoculated i.m. Mortality after intra-abdominal or s.c. inoculation of cells from all four lymphomas was similar to that after i.m. inoculations. Chickens inoculated with syngeneic UG2 cells by the i.v. route were significantly protected against a subsequent i.m. challenge with the same or other syngeneic lymphoma cells. However, UG2 lymphoma cells were highly lethal when inoculated i.v. into birds previously treated with the antimacrophage agent carageenan or immunosuppressed by neonatal treatment with cyclophosphamide. Thus, UG2 cells are distinctive in that, when inoculated i.v., they do not cause death of syngeneic hosts but instead induce resistance to a lethal challenge.