Abstract Rous sarcoma cells even in large numbers grow poorly when spread on the surface of a tissue culture dish unless normal cells are present in the majority. The Rous sarcoma cells can however grow to a much greater extent than normal cells when suspended in soft agar. Conditioned medium obtained from normal cells can support the growth of Rous sarcoma cells. For the first few days after infection the cells continue to release a substance as they had before infection, which enhances the growth of small numbers of normal chick embryo cells. When the infected cells undergo transformation however, they release a substance which, at high concentrations, inhibits the growth of small numbers of normal cells. They also continue to produce the growth enhancing substance, but its presence can only be detected when the effect of the inhibitory material is removed by dilution.