Diffusion chambers containing EBV-bearing human lymphoblastoid cells were placed temporarily within the peritoneal cavity of BALB/c mice with Rauscher leukemia. Reestablished human leukocyte cultures became dually infected with C type and EBV particles and have continuously produced abundant C type particles. These virus particles were nonleukemogenic when inoculated into BALB/c mice but afforded some protection against challenge infection with spleen-derived leukemogenic RLV.