[Control of cell proliferation in vitro. I. Relation of proliferation to blood serum factors].

The population of L-cells sensitive to the cytotoxic action of ethidium bromide (EB) has at least 2 cell types differing in some growth characteristics and in their dependence on serum factors, controlling saturation density of culture. L-cells, resistant to EB, differ from sensitive cells in many features studied and reveal the ability for a continuous and, probably, unlimited multiplication in serum-free medium.