Neoplastic fibroblasts sensitive to the growth inhibition by homologous cells but insensitive to inhibition by parent normal cells

3H-thymidine labelling and autoradiography were used to compare density dependent inhibition of growth in the cultures of two transformed lines of hamster fibroblasts and in primary cultures of their parent normal cells. Similar manifestations of density dependent inhibition were found in the isolated cultures of normal and neoplastic cells: at saturation densities these cultures had low labelling indices; these indices considerably increased when the cells migrated into the wound from the dense sheet, prelabelled cells seeded on the dense sheets of unlabelled homologous cells did not proliferate. However, proliferation of neoplastic cells was not inhibited when they were seeded on the dense sheet of normal fibroblasts. Thus, neoplastic hamster fibroblasts of both lines retained sensitivity to the inhibiting effect of homologous neoplastic cells but completely lost sensitivity to the inhibiting effect of normal fibroblasts. The possible significance of this selective loss of the sensitivity to normal cells is discussed briefly.