When weanling (SJL/J x C57BL/1)F(1) hybrid mice were given five weekly-injections of small doses of viable SJL/J spleen cells, so as to induce a graft-versus-host reaction (GVHR), reticulum cell sarcomas were induced in all of the host mice by the 40th day after the first cell injection. Such tumors, on transplantation, were accepted by syngeneic (SJL/J x C57BL/1)F(1) and C57BL/1 hosts, but not by SJL/J or NZB mice. Cell-free extracts of SJL/J spleens injected into similar hybrids resulted in identical tumors in all hosts within the same period; the transplantation characteristics were also similar. Normal (SJL/J x C57BL/1)F(1) hybrids as well as similar hybrids injected with SJL/J liver or syngeneic F(1) spleen cells did not develop tumors. Cell-free preparations of eight tumors induced in F(1)'s by viable SLJ/J spleen cells were injected into newborn (C57BL/1 x A)F(1) and C57BL/1 mice: tumors were induced, with seven of eight tumor preparations, with a latent period of 33-49 days. Such tumors were lymphosarcomas, and, in the case of (C57BL/1 x A)F(1) hosts, further transplantation revealed that they were antigenically C57BL/1 tumors. These experiments provide conclusive evidence for a viral etiology of GVHR-induced tumors. Furthermore, tumor induction in the GVHR does not appear to depend specifically on an immunological mechanism but is most probably due to release or activation of a sufficient quantity of oncogenic virus within a certain time period in a highly susceptible host. Comparison with radiation induction of viral leukemia in mice revealed similarities in regard to optimal host age and the spacing of administration of the tumor-inducing agent. SJL/J mice carry a type C virus which causes a high incidence of spontaneous Hodgkin-like tumors by 1 yr of age; C57BL/1 mice do not develop lymphomas spontaneously but carry a latent leukemogenic virus. Their hybrid also has a low incidence of spontaneous lymphomas. Based on the results of these and previous experiments, the viruses of these strains of mice appear to be highly synergistic in tumor induction in the GVHR. The SJL/J virus is a powerful oncogenic agent. The C57BL/1 virus may be a helper virus to the SJL/J, but is a more powerful determinant of the antigenic composition of the induced tumors. This suggests that the virus of C57BL/1 mice, when activated, is capable of controlling the C57BL/1 genome. Because of the ease and rapidity of viral tumor induction, the SJL/J and C57BL/1 strains of mice, with their F(1) hybrid, should be useful for further study of the mechanisms controlling induction of such tumors.