An EB virus-carrying lymphoblastoid cell line established from peripheral blood of a patient with SMON was super-infected in vitro with murine Rauscher leukemia virus. Since then, the cell line has been dually infected with both viruses for more than 1.5 years, as repeatedly demonstrated by the presence of herpes type virus particles and C type virus particles. The C type virus particles liberated from the human cultured leukocytes were no longer leukemogenic when inoculated into otherwise susceptible BALB/c mice, but this procedure rendered them considerably resistant to challenge infection with leukemogenic Rauscher leukemia virus that had been maintained by passage through BALB/c mice. The infectivity of the human cell-derived C type virus to other lymphoblastoid cell lines established from different patients and to primary BALB/c mouse embryo cells could not be demonstrated by the method employed. The implication of these findings is discussed in the light of possible viral etiology of human malignancy.