Growth Characteristics of Virus-transformed Cells Maximum Population Density, Inhibition by Normal Cells, Serum Requirement, Growth in Soft Agar, and Xenogeneic Transplantability*

I t has been shown that strains of normal cells which attain relatively low population densities in culture (contact inhibition of growth) inhibit each other in mixed culture (1). This interstrain inhibition is not species-specific (2). Cultured cancer cells, as well as heteroploid cells which arise "spontaneously" in culture, have in varying degree escaped from contact inhibition of growth, and under identical growth conditions attain a higher population density than their diploid counterparts. Such heteroploid cells are similarly insensitive to the growth inhibitory effects of other cell types, and the degree to which they have escaped from growth inhibition in such mixed cultures usually (but not invariably) parallels their maximum growth potential in pure culture (2). Most virus-transformed cells resemble natural cancer cells and spontaneous heteroploid transformants in that they have in varying degrees escaped from contact inhibition of growth, and attain higher population densities than the parent cell; some have an enhanced capacity to form colonies in soft agar (3, 4); many are tumorigenic; and chick embryo cells transformed by Rous sarcoma