Human 293 cells become transiently positive for PERV-C, and productively infected with MLV, following cocultivation with chimeric bone marrow samples that contain murine cells. Human 293 cells were directly cocultured with chimeric bone marrow containing mouse + pig (M/P), mouse + pig + human (M/P/H), or mouse + human (M/H) cells. To determine the replication competence of the viruses in this primary culture (G1), a cell-free supernatant preparation was used to further challenge secondary uninfected 293 cells (G2). Cells were tested by PCR for the presence of MLV and PERV-C sequences.