SUMMARY

During serial passage of mouse embryo fibroblasts in vitro it was found that an established line of cells was more susceptible to murine sarcoma virus trans-formation than early passages of the cells from which it was derived. An explana-tion can be made in terms of the growth rates of the cells involved, although primary cells were more susceptible to murine sarcoma virus than would be expected from their rate of growth. Non-dividing (amitotic) cells were not suscep-tible to murine sarcoma virus transformation