Studies on the loss of growth inhibition in cells infected with Rous sarcoma virus

The loss of density‐dependent and anchorage‐dependent growth was studied in cells infected with RSV. Transformed cells, but not uninfected cells, will proliferate in soft gel suspension and amongst crowded normal cells. A large proportion of cells freshly infected with RSV acquired the capacity to proliferate in soft agar suspension even when placed in suspension immediately after infection. Cell transformation and viral replication were inhibited by plating freshly infected cells on to crowded mouse embryo cells or 3T3 cells. The inhibition of cell transformation depended on close proximity to crowded mouse cells spread on a solid substratum. Cells became released from density‐dependent transformation within 12 h following infection. The change in proliferative behaviour of transformed cells was not influenced by the quantity or envelope properties of virus released from the cell surface. Pre‐infection with RAV1 did not affect density‐dependent cell transformation by RSV. Freshly infected cells held in the inhibited state for 6 days were able to transform when replated to sparse conditions. It was concluded that cell transformation depended on cell proliferation but that infection with RSV could not initiate the cell division cycle in crowded monolayers.